[Bluetooth] Check if DLC is still attached to the TTY

If the DLC device is no longer attached to the TTY device, then it
makes no sense to go through with changing the termios settings.
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

net/bluetooth/rfcomm/tty.c

@@ -765,6 +765,9 @@ static void rfcomm_tty_set_termios(struct tty_struct *tty, struct termios *old)
 
 	BT_DBG("tty %p termios %p", tty, old);
 
+	if (!dev)
+		return;
+
 	/* Handle turning off CRTSCTS */
 	if ((old->c_cflag & CRTSCTS) && !(new->c_cflag & CRTSCTS)) 
 		BT_DBG("Turning off CRTSCTS unsupported");