net/mlx5: fix Tx meta width for modify field flow rule

net/mlx5: fix Tx meta width for modify field flow rule

Register C is used for the metadata within NIC Rx domain.
And its width can vary from 0 to 32 bits depending on
its kernel usage. But it is not the case within NIC Tx domain,
register A is always 32 bits there. Fix metadata width detection
for the modify_field flow API within NIC Tx domain.

Fixes: 6d5735c1cba2 ("net/mlx5: fix meta register conversion for extensive mode")
Cc: stable@dpdk.org
Signed-off-by: Alexander Kozyrev <akozyrev@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>