Fix Ethertype check for push MPLS action

Expect ethertype 8847 (MPLS unicast) or 8848 (MPLS multicast), not
88A8 (VLAN).  This is now consistent with the check in linc_us4_actions.

This closes FlowForwarding/LINC-Switch#209.
Merge pull request #215 from legoscia/validate_push_mpls

Fix Ethertype check for push MPLS action