ath9k_hw: fix gentimer callback without overflow handler

ath9k_hw: fix gentimer callback without overflow handler

If a gentimer has both the trigger and the overflow bits set, only
mask out the trigger bit if an overflow handler is present.

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