mmc: sdio: Fix PM_SLEEP related build warnings

Power management callbacks defined by SIMPLE_DEV_PM_OPS are only used if
the PM_SLEEP Kconfig symbol has been defined. If not, the compiler will
complain about them being unused. However, since the callback for this
driver doesn't do anything it can just as well be dropped.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Chris Ball <cjb@laptop.org>