ath9k: Fix kernel panic in AR2427

Kernel panic occurs just after AR2427 establishes connection with AP.
Unless aggregation is enabled we don't initialize the TID structure.
Thus accesing the elements of the TID structure when aggregation is
disabled, leads to NULL pointer dereferencing.

[  191.320358] Call Trace:
[  191.320364]  [<fd250ea7>] ? ath9k_tx+0xa7/0x200 [ath9k]
[  191.320376]  [<fd1ec7fc>] ? __ieee80211_tx+0x5c/0x1e0 [mac80211]
[  191.320386]  [<fd1edd2b>] ? ieee80211_tx+0x7b/0x90 [mac80211]
[  191.320395]  [<fd1edddd>] ? ieee80211_xmit+0x9d/0x1d0 [mac80211]
[  191.320401]  [<c014218f>] ? wake_up_state+0xf/0x20
[  191.320405]  [<c015dbc8>] ? signal_wake_up+0x28/0x40
[  191.320410]  [<c012a578>] ? default_spin_lock_flags+0x8/0x10
[  191.320420]  [<fd1ee308>] ? ieee80211_subif_start_xmit+0x2e8/0x7c0
[mac80211]
[  191.320425]  [<c058f905>] ? do_page_fault+0x295/0x3a0
[  191.320431]  [<c04c4a3d>] ? dev_hard_start_xmit+0x1ad/0x210
[  191.320436]  [<c04d96b5>] ? sch_direct_xmit+0x105/0x170
[  191.320445]  [<fd1f161a>] ? get_sta_flags+0x2a/0x40 [mac80211]
[  191.320449]  [<c04c780f>] ? dev_queue_xmit+0x37f/0x4b0
[  191.320452]  [<c04d75b0>] ? eth_header+0x0/0xb0
[  191.320456]  [<c04cc479>] ? neigh_resolve_output+0xe9/0x310
[  191.320461]  [<c053d295>] ? ip6_output_finish+0xa5/0x110
[  191.320464]  [<c053e354>] ? ip6_output2+0x134/0x250
[  191.320468]  [<c053f7dd>] ? ip6_output+0x6d/0x100
[  191.320471]  [<c0559665>] ? mld_sendpack+0x395/0x3e0
[  191.320475]  [<c0557f81>] ? add_grhead+0x31/0xa0
[  191.320478]  [<c055a83c>] ? mld_send_cr+0x1bc/0x2b0
[  191.320482]  [<c01535d9>] ? irq_exit+0x39/0x70
[  191.320485]  [<c055a940>] ? mld_ifc_timer_expire+0x10/0x40
[  191.320489]  [<c015b92e>] ? run_timer_softirq+0x13e/0x2c0
[  191.320493]  [<c0103a30>] ? common_interrupt+0x30/0x40
[  191.320498]  [<c055a930>] ? mld_ifc_timer_expire+0x0/0x40
[  191.320502]  [<c0153358>] ? __do_softirq+0x98/0x1b0
[  191.320506]  [<c01534b5>] ? do_softirq+0x45/0x50
[  191.320509]  [<c0153605>] ? irq_exit+0x65/0x70
[  191.320513]  [<c05917dc>] ? smp_apic_timer_interrupt+0x5c/0x8b
[  191.320516]  [<c0103df1>] ? apic_timer_interrupt+0x31/0x40
[  191.320521]  [<c016007b>] ? k_getrusage+0x12b/0x2f0
[  191.320525]  [<c039e384>] ? acpi_idle_enter_simple+0x117/0x148
[  191.320529]  [<c04a20da>] ? cpuidle_idle_call+0x7a/0x100
[  191.320532]  [<c01021d4>] ? cpu_idle+0x94/0xd0
[  191.320536]  [<c057ab88>] ? rest_init+0x58/0x60
[  191.320541]  [<c07a58ec>] ? start_kernel+0x351/0x357
[  191.320544]  [<c07a53c7>] ? unknown_bootoption+0x0/0x19e
[  191.320548]  [<c07a50aa>] ? i386_start_kernel+0xaa/0xb1
[  191.320550] Code: 03 66 3d 00 03 0f 84 7c 02 00 00 83 c3 18 0f b6 03
8b 4d e0 89 c3 83 e3 0f 6b c3 48 89 5d d8 8d 04 06 8d 50 0c 89 55 d0 8b
40 20 <8b> 00 3b 01 0f 85 8e 02 00 00 f6 47 20 40 0f 84 29 ff ff ff 8b
[  191.320634] EIP: [<fd2586d4>] ath_tx_start+0x474/0x770 [ath9k] SS:ESP
0068:c0761a90
[  191.320642] CR2: 0000000000000000
[  191.320647] ---[ end trace 9296ef23b9076ece ]---
[  191.320650] Kernel panic - not syncing: Fatal exception in interrupt

Cc: stable@kernel.org
Signed-off-by: Mohammed Shafi Shajakhan <mshajakhan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>