net/mlx5: Support enable_eth devlink dev param

Enable user to disable Ethernet auxiliary device so that when it is not
required, user can disable it.

For example,

$ devlink dev param set pci/0000:06:00.0 \
              name enable_eth value false cmode driverinit
$ devlink dev reload pci/0000:06:00.0

At this point devlink instance do not create mlx5_core.eth.2 auxiliary
device for the Ethernet functionality.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>