net/mlx5: fix build on Arm

net/mlx5: fix build on Arm

On some ARM environment, the below compilation error will be seen

dpdk/drivers/net/mlx5/mlx5_flow_dv.c: In function
'flow_dv_translate_item_nvgre':
/tmp/dpdk/drivers/net/mlx5/mlx5_flow_dv.c:785:22: error: pointer targets
in initialization differ in signedness [-Werror=pointer-sign]
  const char *tni_v = nvgre_v->tni;

The reason for this error is that nvgre_v->tni is defined as byte array
in size of 3B. However the code in the function iterate till the 4B in
order to copy/set also the subsequent field after it (flow_id)

Fixing by pointing to this struct from a different pointer.

Fixes: fc2c498ccb94 ("net/mlx5: add Direct Verbs translate items")

Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Ori Kam <orika@mellanox.com>