mt7621-mmc: char * array declaration might be better as static const

Issue reported by checkpatch.pl.

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>