staging: mt7621-mmc: Fix dereference before check in msdc_drv_pm

In the msdc_drv_pm function the variable mmc is dereferenced before
checked. Reordering fixes that.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>