mmc: sdio: ensure mmc_sdio_detect is powered

To prevent an erroneous removal of the card, make sure
the device is powered when it is mmc_sdio_detect()ed.

This is required since mmc_sdio_detect may be invoked
while the device is runtime suspended (e.g., MMC core
is rescanning when system comes out of suspend).

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Tested-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Chris Ball <cjb@laptop.org>