isdn: mark expected switch fall-throughs

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

Warning level 2 was used: -Wimplicit-fallthrough=2

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/hardware/mISDN/hfcpci.c

@@ -1296,6 +1296,7 @@ mode_hfcpci(struct bchannel *bch, int bc, int protocol)
 	case (-1): /* used for init */
 		bch->state = -1;
 		bch->nr = bc;
+		/* fall through */
 	case (ISDN_P_NONE):
 		if (bch->state == ISDN_P_NONE)
 			return 0;