net/mlx5: fix NVGRE matching

NVGRE has a GRE header with c_rsvd0_ver value 0x2000 and protocol
value 0x6558.
These should be matched when item_nvgre is provided.

This patch adds validation function of NVGRE item.
It also updates the translate function of NVGRE item, to add the
required values, if they were not specified.

Original work by Xiaoyu Min <jackmin@mellanox.com>

Fixes: fc2c498ccb94 ("net/mlx5: add Direct Verbs translate items")
Cc: stable@dpdk.org
Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Xiaoyu Min <jackmin@mellanox.com>