Enable runtime PM for the wl1271 SDIO device.

We request power whenever the WLAN interface is brought up,
and release it after the WLAN interface is taken down.

As a result, power is released immediately after probe returns,
since at that point power has not been explicitly requested yet
(i.e. the WLAN interface is still down).
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Acked-by: Luciano Coelho <luciano.coelho@nokia.com>
Tested-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>