i40e: stop VF rings

Explicitly stop the rings belonging to each VF when disabling SR-IOV.
Even though the VFs were gone, and the associated VSIs were removed, the
rings were not stopped, and in some circumstances the hardware would
continue to access the memory formerly used by the rings, causing memory
corruption or DMAR errors, both of which would lead to general malaise
of the kernel.

To relieve this condition, explicitly stop all the rings associated with
each VF before releasing its resources.

Change-ID: I78c05d562c66e7b594b7e48d67860f49b3e5b6ec
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>