wl1251: add runtime PM support for SDIO

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>