ath9k: use a timer to put hardware into full sleep

When operating in client mode, the short period of time between scanning
and associating is often enough to put the hardware through several
FULL-SLEEP <-> AWAKE transitions, each wakeup requiring a reset to fully
recover the hardware.
This is completely unnecessary and can easily be avoided by deferring
the switch to full sleep.

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