wl12xx: don't bail if mmc isn't MMC_CAP_POWER_OFF_CARD

If our SDIO function has its runtime PM disabled, don't try to
manipulate its runtime PM status at all. This way we can still
power on cards plugged to mmc hosts that are not MMC_CAP_POWER_OFF_CARD.

Reported-and-tested-by: Tim Yamin <tim@kangatronix.co.uk>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>