i40e: stop the service task at shutdown

Stop the service task in the shutdown handler, preventing it from
accessing the admin queue after it had been closed. This fixes a panic
that could occur when the system was shut down with a lot of VFs
enabled.

Change-ID: I286735e3842de472385bbf7ad68d30331e508add
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>