PCI: designware: Make MSI ISR shared IRQ aware

On i.MX6 the host controller MSI IRQ is shared with PCI legacy INTD.  Make
sure we don't bail too early from the IRQ handler.

The issue is fairly theoretical as it would require a system setup with a
PCIe switch where one connected device is using legacy INTD and another one
using MSI, but better fix it now.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Richard Zhu <r65037@freescale.com>