Bluetooth: Mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

This patch fixes the following warnings:

net/bluetooth/rfcomm/core.c:479:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
net/bluetooth/l2cap_core.c:4223:6: warning: this statement may fall through [-Wimplicit-fallthrough=]

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enabling
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/l2cap_core.c

@@ -4265,6 +4265,7 @@ static inline int l2cap_config_rsp(struct l2cap_conn *conn,
 				goto done;
 			break;
 		}
+		/* fall through */
 
 	default:
 		l2cap_chan_set_err(chan, ECONNRESET);

net/bluetooth/rfcomm/core.c

@@ -483,6 +483,7 @@ static int __rfcomm_dlc_close(struct rfcomm_dlc *d, int err)
 		/* if closing a dlc in a session that hasn't been started,
 		 * just close and unlink the dlc
 		 */
+		/* fall through */
 
 	default:
 		rfcomm_dlc_clear_timer(d);