i40evf: enable interrupt 0 appropriately

Don't enable vector 0 in the ISR, just schedule the adminq task and let
it enable the vector. This prevents the task from being called
reentrantly. Make sure that the vector is enabled on all exit paths of
the adminq task, including error exits.

Change-ID: I53f3d14f91ed7a9e90291ea41c681122a5eca5b5
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Acked-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>