/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>

drivers/staging/dgrp/dgrp_tty.c

@@ -2798,6 +2798,7 @@ static int dgrp_tty_ioctl(struct tty_struct *tty, unsigned int cmd,
 		}
 
 		/* pretend we didn't recognize this */
+		/* fall-through */
 
 	case DIGI_SETA:
 		return dgrp_tty_digiseta(tty, (struct digi_struct *) arg);