Add runtime PM support, similar to how it's done for wl1271.
This allows to power down the card when the driver is loaded but
network is not in use.

Cc: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Acked-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>