When the timer_next argument to ath9k_gen_timer_start is behind the tsf value,
tsf + timer_period is used, which is what ath_btcoex_period_timer was setting
it to.

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