i40evf: fix oops in watchdog handler

The Tx watchdog handler runs in interrupt context, so it would cause an
oops when sending an admin queue message to request a reset, because the
admin queue functions use spinlocks.

Instead, set a flag and let the reset task handle sending the request.

Change-ID: I65879470b72963d9c308edfb8f45ac4fbba2c14f
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Catherine Sullivan <catherine.sullivan@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>