net/mlx4_core: Prevent VF from changing port configuration

Added wrapper to the ACCESS_REG command for handling guest HW
registers access, preventing write operations, but do allow reads.

This will prevent SRIOV guests to change port PTYS configuration,
such as speed/advertised link modes.

Fixes: adbc7ac5

 ('net/mlx4_core: Introduce ACCESS_REG CMD [...]')
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>