i40evf: check rings before freeing resources

If the driver gets unloaded during reset recovery, it's possible
that it will attempt to free resources when they're already free.

Add a check to make sure that the Tx and Rx rings actually exist
before dereferencing them to free resources.

Change-ID: I4d2b7e9ede49f634d421a4c5deaa5446bc755eee
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>