net/enic: support GTP header flow matching

net/enic: support GTP header flow matching

The GTP, GTP-U, GTP-C header fields can be matched, however NIC does not
support GTP tunneling so no items after the GTP header can be specified.
If a GTP-U or GTP-C item is specified without a preceding UDP item, the
UDP destination port is implicitly matched. For GTP, the destination UDP
port must be specified but its value is not enforced.

Signed-off-by: John Daley <johndale@cisco.com>
Reviewed-by: Hyong Youb Kim <hyonkim@cisco.com>