net/mlx5_core: Set/Query port MTU commands

Introduce set/Query low level functions to access MTU in hardware. To be
used by the netdev.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>