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().

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>