/dgrp: Fixed warning by adding fallthrough comment

This patch solves the warning "possible switch case/default not preceded
by break or fallthrough comment" for the file
/drivers/staging/dgrp/dgrp_tty.c

Signed-off-by: Andreea-Cristina Bernat <bernat.ada@gmail.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>