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

Add missing return statement for CONFIG_BUG=n.
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/8021q/vlan.h

@@ -121,6 +121,7 @@ static inline unsigned int vlan_proto_idx(__be16 proto)
 		return VLAN_PROTO_8021AD;
 	default:
 		BUG();
+		return 0;
 	}
 }