mmc: sdio: fix runtime PM path during driver removal

After commit e1866b33 "PM / Runtime: Rework runtime PM handling
during driver removal" was introduced, the driver core stopped
incrementing the runtime PM usage counter of the device during
the invocation of the ->remove() callback.

This indirectly broke SDIO's runtime PM path during driver removal,
because no one calls _put_sync() anymore after ->remove() completes.

This means that the power of runtime-PM-managed SDIO cards is kept
high after their driver is removed (even if it was powered down
beforehand).

Fix that by directly calling _put_sync() when the last usage
counter is downref'ed by the SDIO bus.

Reported-and-tested-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Chris Ball <cjb@laptop.org>