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.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>