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: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>