nfp: Replace ndo_add/del_vxlan_port with ndo_add/del_udp_enc_port

This change replaces the network device operations for adding or removing a
VXLAN port with operations that are more generically defined to be used for
any UDP offload port but provide a type.  As such by just adding a line to
verify that the offload type is VXLAN we can maintain the same
functionality.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>