ath9k_hw: enable interrupts for beacon tx completion events

Not doing so could cause the tx status queue to overflow during longer
periods of time without non-beacon tx. These events are also required
for proper drv_tx_last_beacon handling.

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