staging: mt7621-mmc: Fix comparison to NULL

Replace comparisons of a variable 'x' to NULL with
either 'x' or '!x'.
Issues found with checkpatch.pl

Signed-off-by: Dafna Hirschfeld <dafna3@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>