ath9k: do not access hw registers in FULL SLEEP

The opmode recalculation is accessing hw registers.
When it is called from remove interface callback and
if there are no vifs present then hw is moved to FULL SLEEP
by radio disable. So use power save wrappers before
accessing hw registers in calculating opmode state.

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>