In AHCI_IRQ_MODE_AFTER mode do not clear interrupts below.

This is probably a NOP change since IS register is not activery used for
interrupts below the shared, but it looked odd to clear interrupts we did
not handle.

sys/dev/ahci/ahci.c

@@ -466,6 +466,7 @@ ahci_intr(void *data)
 	} else {	/* AHCI_IRQ_MODE_AFTER */
 		unit = irq->r_irq_rid - 1;
 		is = ATA_INL(ctlr->r_mem, AHCI_IS);
+		is &= (0xffffffff << unit);
 	}
 	/* CCC interrupt is edge triggered. */
 	if (ctlr->ccc)