net: vlan: fix up vlan_proto_idx() for CONFIG_BUG=n

Add missing return statement for CONFIG_BUG=n.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/8021q/vlan.h b/net/8021q/vlan.h
index abc9cb6..ba5983f 100644
--- a/net/8021q/vlan.h
+++ b/net/8021q/vlan.h
@@ -121,6 +121,7 @@
 		return VLAN_PROTO_8021AD;
 	default:
 		BUG();
+		return 0;
 	}
 }