net/mlx5: fix meta field size for flow modify

net/mlx5: fix meta field size for flow modify

The modify_field flow API assumes that the META item is 32 bits wide.
But the C register that is used for META item can be 16 or 32 bits
wide depending on kernel and firmware configurations.
Take this into consideration and use the appropriate META width.

Fixes: 641dbe4fb053 ("net/mlx5: support modify field flow action")
Cc: stable@dpdk.org
Signed-off-by: Alexander Kozyrev <akozyrev@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>