i40evf: null out ring pointers on free

Since we check these ring pointers to make sure we don't double-allocate
or double-free the rings, we had better null them out after we free
them. In very rare cases this can cause a panic if the driver is removed
during reset recovery.

Change-ID: Ib06eb4910a3058275c8f7ec5ef7f45baa4674f96
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>