staging: mt7621-pinctrl: replace seq_printf with seq_puts

For a constant format without additional arguments, use seq_puts()
instead of seq_printf() fixing also the following checkpatch.pl
warning: 'Prefer seq_puts to seq_printf'

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>