staging: sep: Add fallthrough comment

This patch solves the warning "possible switch/case default not preceded by
break or fallthrough comment".
Signed-off-by: Andreea-Cristina Bernat <bernat.ada@gmail.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>

drivers/staging/sep/sep_main.c

@@ -3073,6 +3073,7 @@ static long sep_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 		dev_dbg(&sep->pdev->dev,
 			"[PID%d] SEP_IOCPREPAREDCB start\n",
 			current->pid);
+		/* fall-through */
 	case SEP_IOCPREPAREDCB_SECURE_DMA:
 		dev_dbg(&sep->pdev->dev,
 			"[PID%d] SEP_IOCPREPAREDCB_SECURE_DMA start\n",