net/mlx5: Fix wrong size allocation for QoS ETC TC regitster

The driver allocates wrong size (due to wrong struct name) when issuing
a query/set request to NIC's register.

Fixes: d8880795

 ("net/mlx5e: Implement DCBNL IEEE max rate")
Signed-off-by: Shay Agroskin <shayag@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>