net/mlx4_en: Adding support of turning off link autonegotiation via ethtool

This feature will allow the user to disable auto negotiation
on the port for mlx4 devices while setting the speed is limited
to 1GbE speeds.
Other speeds will not be accepted in autoneg off mode.

This functionality is permitted providing that the firmware
is compatible with this feature.
The above is determined by querying a new dedicated capability
bit in the device.

Signed-off-by: Ariel Levkovich <lariel@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>