igb: Teardown SR-IOV before unregister_netdev()

When the .remove() callback for a PF is called, SR-IOV support for the
device is disabled, which requires unbinding and removing the VFs.
The VFs may be in-use either by the host kernel or userspace, such as
assigned to a VM through vfio-pci.  In this latter case, the VFs may
be removed either by shutting down the VM or hot-unplugging the
devices from the VM.  Unfortunately in the case of a Windows 2012 R2
guest, hot-unplug is broken due to the ordering of the PF driver
teardown.  Disabling SR-IOV prior to unregister_netdev() avoids this
issue.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>