net/mlx5: Reduce dependency on enabled_vfs counter and num_vfs
While enabling SR-IOV, PCI core already checks that if SR-IOV is already
enabled, it returns failure error code.
Hence, remove such duplicate check from mlx5_core driver.
While at it, make mlx5_device_disable_sriov() to perform cleanup of VFs in
reverse order of mlx5_device_enable_sriov().