mmc: core: clear MMC_PM_KEEP_POWER flag on resume

Since the MMC_PM_KEEP_POWER flag should be set on each suspend,
it should also cleared on each resume.

Upon resuming, we have to know if power was kept
(for re-initialization, etc.), so clear it just after resuming.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Chris Ball <cjb@laptop.org>