ath9k: BH shouldn't be enabled when hardirqs are disabled.

ath_tx_complete_buf uses a BH version of spinlock and so releasing
the lock enables BH which is incorrect when called from sta_notify
callback as MAC80211 disables hardirqs before the driver callback
is called.

As ath_tx_complete_buf is shared between user and softirq context
using normal spinlock may not be appropriate. Though the proper
fix would be to cleanup the context properly in the driver code,
this would be an interim fix to avoid kernel warning.

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>