PCI: dwc: Clear MSI interrupt status after it is handled, not before

If the interrupt status is cleared before it is handled, it is possible
that another interrupt will trigger while servicing the previous one.  This
is causing timeouts in some wireless lan cards which use PCIe.

Clear MSI interrupt status after it gets serviced instead of before calling
generic_handler.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-By: Joao Pinto <jpinto@synopsys.com>