net/mlx5: fix validation of VXLAN/VXLAN-GPE specs

net/mlx5: fix validation of VXLAN/VXLAN-GPE specs

Trying to create zero spec for vni wasn't allowed, to
avoid matching all packets from previous layer (udp).
This behavior is incorrect, since VXLAN is being identified
through the outer UDP destination port.

Currently, if the user didn't specify outer UDP destination
port the PMD will automatically match only on  outer
UDP port of 4798, and if the user want to match on some none
standard port he need to specify it explicitly in the rule.

This removes the limitation of vni spec to be able to match any
vni.

Fixes: 23c1d42c7138 ("net/mlx5: split flow validation to dedicated function")
Cc: stable@dpdk.org
Signed-off-by: Raslan Darawsheh <rasland@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>