ath9k: merge reset related functions

reduces unnecessary code duplication. Also takes the sc_pcu_lock within
ath_reset instead of callsites, which makes it possible to always cancel
all queued work items before the reset, possibly fixing a few race
conditions (work items vs reset) along with it.

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