mmc: propagate power save/restore ops return value

Allow power save/restore and their relevant mmc_bus_ops handlers
exit with a return value.

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>