ath9k: mark wmi_event_swba as __packed

The other structures in wmi.h are already marked this way.
Without this marking, we get an unaliged access panic in the tilegx kernel:

Starting stack dump of tid 0, pid 0 (swapper) on cpu 35 at cycle 198675113844
  frame 0: 0xfffffff7103ada90 ath9k_htc_swba+0x120/0x618 [ath9k_htc]
  frame 1: 0xfffffff7103a4b10 ath9k_wmi_event_tasklet+0x1b0/0x270 [ath9k_htc]
  frame 2: 0xfffffff700326570 tasklet_action+0x148/0x298
  [...]

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>