ath9k_hw: clean up generic timer code

- Use generic bitops instead of custom hackery
- Move interrupt enable/disable logic from ath9k to ath9k_hw
- Decouple ISR call from btcoex
- Make the overflow callback optional (to prevent IRQ storms)

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

diff --cc drivers/net/wireless/ath/ath9k/btcoex.c
Simple merge

diff --cc drivers/net/wireless/ath/ath9k/gpio.c
Simple merge

diff --cc drivers/net/wireless/ath/ath9k/hw.c
Simple merge

diff --cc drivers/net/wireless/ath/ath9k/hw.h
Simple merge

diff --cc drivers/net/wireless/ath/ath9k/main.c
Simple merge