i40evf: disable NAPI polling sooner

When closing the interface, disable NAPI polling before any other
activities. This fixes an occasional panic during close caused by the
driver trying to delete and clean rings at the same time.

Change-ID: Ib4d427b13d310258ea85b248d535da70ecf0c1e9
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>