staging: mt7621-mmc: place a check for sscanf

Placed a check for the return value of sscanf. -EINVAL is returned if
the value is anything other than expected. Reported by checkpatch.pl

Signed-off-by: Sumit Pundir <pundirsumit11@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>