mmc: sdio: Add empty bus-level suspend/resume callbacks

Suspend methods provided by SDIO drivers are not supposed to be called by
the PM core.  Instead, when the SDIO core gets to suspend a device's
ancestor, it calls the device driver's suspend routine.  However, the PM
core executes suspend callback routines directly for device drivers whose
bus types don't provide suspend callbacks.  In consequece, because the
SDIO bus type doesn't provide a suspend callback, the SDIO drivers'
suspend routines will be executed by the PM core (which shouldn't
happen).

To prevent this from happening, add empty system suspend/resume callbacks
for the SDIO bus type.

An analogous change had been made already by commit (e841a7c6 mmc: sdio:
Use empty system suspend/resume callbacks at the bus level), but then it
was reverted inadvertently by commit (d8e2ac33 mmc: sdio: Fix PM_SLEEP
related build warnings) that attempted to fix build warnings introduced
by commit e841a7c6.

Reported-by: NeilBrown <neilb@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>