i40evf: stop the watchdog for shutdown

Stop the watchdog during shutdown. Failing to do this causes a log full
of admin queue errors and the occasional hang when the system is shut
down.

Change-ID: Ib2fd11213cca2fa589eb68577e86b1000c23c250
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>