net/mlx5: Access register and MAD IFC commands via mlx5 ifc

Remove old representation of manually created ACCESS_REG/MAD_IFC
commands layout and use mlx5_ifc canonical structures and defines.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>