PCI: pciehp: Return IRQ_NONE when we can't read interrupt status

After 1469d17d ("PCI: pciehp: Handle invalid data when reading from
non-existent devices"), we returned IRQ_HANDLED when we failed to read
interrupt status from the bridge.  I think it's better to return IRQ_NONE,
as we do in other cases where there's no interrupt pending.  This will
facilitate refactoring the loop in pcie_isr(): we'll be able to call the
ISR in a loop as long as it returns IRQ_HANDLED.

Return IRQ_NONE if we couldn't read interrupt status.
Tested-by: NLukas Wunner <lukas@wunner.de>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Reviewed-by: NMika Westerberg <mika.westerberg@linux.intel.com>

drivers/pci/hotplug/pciehp_hpc.c

@@ -561,7 +561,7 @@ static irqreturn_t pcie_isr(int irq, void *dev_id)
 		if (status == (u16) ~0) {
 			ctrl_info(ctrl, "%s: no response from device\n",
 				  __func__);
-			return IRQ_HANDLED;
+			return IRQ_NONE;
 		}
 
 		/*