net/mlx4_core: Avoid setting ports to auto when only one port type is supported

When only one port type is supported, it should be read only.
We reject changing requests, even to the auto sense mode.

Fixes: 27bf91d6 ("mlx4_core: Add link type autosensing")
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>