net: ax25: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ax25/af_ax25.c

@@ -850,6 +850,7 @@ static int ax25_create(struct net *net, struct socket *sock, int protocol,
 		case AX25_P_ROSE:
 			if (ax25_protocol_is_registered(AX25_P_ROSE))
 				return -ESOCKTNOSUPPORT;
+			break;
 #endif
 		default:
 			break;