net: ethernet: mlx4: remove unused struct 'mlx4_port_config'

'mlx4_port_config was added by
commit ab9c17a0 ("mlx4_core: Modify driver initialization flow to
accommodate SRIOV for Ethernet")
but remained unused.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>