ofp-actions: Correct pop MPLS ethtype as consistency test

ofp-actions: Correct pop MPLS ethtype as consistency test

Correct pop MPLS ethtype consistency check to verify that
the packet has an MPLS ethtype before the pop action rather than after:
an MPLS ethtype is a pre-condition but not a post-condition of pop MPLS.

With this change the consistency check in ofpact_check__()
becomes consistent with that in ofpact_from_nxast().

This was found using Ryu tests via the new make check-ryu target.
It allows all of the "POP_MPLS"[1] tests to pass where they previously
failed.

[1] http://osrg.github.io/ryu-certification/switch/ovs

Cc: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>