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>

drivers/net/wireless/ath/ath9k/main.c

@@ -508,6 +508,9 @@ void ath9k_tasklet(unsigned long data)
 		wake_up(&sc->tx_wait);
 	}
 
+	if (status & ATH9K_INT_GENTIMER)
+		ath_gen_timer_isr(sc->sc_ah);
+
 	ath9k_btcoex_handle_interrupt(sc, status);
 
 	/* re-enable hardware interrupt */