ath5k: fix reset race

To prepare for reset ath5k should finish all asynchronous tasks. At
first, it disables the interrupt generation, then it waits for the
interrupt handler and tasklets completion, and then proceeds to the HW
configuration update. But it does not consider that the interrupt
handler or tasklet re-enables the interrupt generation. And we fall in a
situation when ath5k assumes that interrupts are disabled, but it is
not.

This can lead to different consequences, such as reception of the frame,
when we do not expect it. Under certain circumstances, this can lead to
the following warning:

  WARNING: at ath5k/base.c:589 ath5k_tasklet_rx+0x318/0x6ec [ath5k]()
  invalid hw_rix: 1a
  [..]
  Call Trace:
  [<802656a8>] show_stack+0x48/0x70
  [<802dd92c>] warn_slowpath_common+0x88/0xbc
  [<802dd98c>] warn_slowpath_fmt+0x2c/0x38
  [<81b51be8>] ath5k_tasklet_rx+0x318/0x6ec [ath5k]
  [<8028ac64>] tasklet_action+0x8c/0xf0
  [<80075804>] __do_softirq+0x180/0x32c
  [<80196ce8>] irq_exit+0x54/0x70
  [<80041848>] ret_from_irq+0x0/0x4
  [<80182fdc>] ioread32+0x4/0xc
  [<81b4c42c>] ath5k_hw_set_sleep_clock+0x2ec/0x474 [ath5k]
  [<81b4cf28>] ath5k_hw_reset+0x50/0xeb8 [ath5k]
  [<81b50900>] ath5k_reset+0xd4/0x310 [ath5k]
  [<81b557e8>] ath5k_config+0x4c/0x104 [ath5k]
  [<80d01770>] ieee80211_hw_config+0x2f4/0x35c [mac80211]
  [<80d09aa8>] ieee80211_scan_work+0x2e4/0x414 [mac80211]
  [<8022c3f4>] process_one_work+0x28c/0x400
  [<802df8f8>] worker_thread+0x258/0x3c0
  [<801b5710>] kthread+0xe0/0xec
  [<800418a8>] ret_from_kernel_thread+0x14/0x1c

Fix this issue by adding a new status flag, which forbids to re-enable
the interrupt generation until the HW configuration is completed.

Note: previous patch, which reorders the Rx disable code helps to avoid
the above warning, but not fixes the root cause of unexpected frame
receiving.

CC: Jiri Slaby <jirislaby@gmail.com>
CC: Nick Kossifidis <mickflemm@gmail.com>
CC: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Reported-by: Christophe Prevotaux <cprevotaux@nltinc.com>
Tested-by: Christophe Prevotaux <cprevotaux@nltinc.com>
Tested-by: Eric Bree <ebree@nltinc.com>
Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>