staging: mt7621-mmc: Replace seq_printf with seq_puts where possible

Replace seq_printf with seq_puts when the string is const.
Fix checkpatch warning: PREFER_SEQ_PUTS

Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>