ixgbe(4): Fix panic triggered by assertion from interrupt

r344162 exposed a bug in one of ixgbe's interrupt filters; they are never
supposed to return 0. Fix the interrupt filter to return the proper nonzero
return value.

Reported by:	Oleg Ginzburg <olevole@olevole.ru>
MFC after:	1 week
Sponsored by:	Intel Corporation

sys/dev/ixgbe/if_ix.c

@@ -2064,7 +2064,7 @@ ixgbe_msix_que(void *arg)
 
 	/* Protect against spurious interrupts */
 	if ((ifp->if_drv_flags & IFF_DRV_RUNNING) == 0)
-		return 0;
+		return (FILTER_HANDLED);
 
 	ixgbe_disable_queue(adapter, que->msix);
 	++que->irqs;