ath9k_hw: optimize interrupt mask changes

OProfile showed that ath9k was spending way too much time in
ath9k_hw_set_interrupts. Since most of the interrupt mask changes only
need to globally enable/disable interrupts, it makes sense to split
this part into separate functions, replacing all calls to
ath9k_hw_set_interrupts(ah, 0) with ath9k_hw_disable_interrupts(ah).

ath9k_hw_set_interrupts(ah, ah->imask) only gets changed to
ath9k_hw_enable_interrupts(ah), whenever ah->imask was not changed
since the point where interrupts were disabled.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>