ath9k_hw: use a software timer for btcoex no_stomp_timer

TSF accuracy is not needed here, and there is only one usable generic
timer that is supported by all chips and uses the primary TSF counter.

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