mt7621-mmc: Fix some coding style issues

Fix some comment-related issues reported by checkpatch.pl:
- Block comments use * on subsequent lines
- Block comments use a trailing */ on a separate line

Signed-off-by: Jona Crasselt <jona.crasselt@fau.de>
Signed-off-by: Felix Windsheimer <felix.windsheimer@fau.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>