wl1271: sdio: enable runtime PM

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>