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>