i40e: Don't disable SR-IOV when VFs are assigned

When VFs are assigned to active VMs and we disable SR-IOV out from under them,
bad things happen. Currently, the VM does not crash, but the VFs lose all
resources and have no way to get them back.

Add an additional check for when the user is disabling through sysfs, and add a
comment to clarify why we check twice.

Change-ID: Icad78eef516e4e1e4a87874d59132bc3baa058d4
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>