wl1271: propagate set_power's return value

Make it possible for the set power method to indicate a
success/failure return value. This is needed to support
more complex power on/off operations such as SDIO
power manipulations.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Acked-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>