ath9k: improve suspend/resume reliability

Ensure that drv_start() always returns true, as a failing hw start usually
eventually leads to crashes when there's still a station entry present.
Call a power-on reset after a resume and after a hw reset failure to bring
the hardware back to life again.

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