PCI/DPC: Clear interrupt status in interrupt handler top half

The generic IRQ handling code ensures that an interrupt handler runs with
its interrupt masked or disabled.  If the interrupt is level-triggered, the
interrupt handler must tell its device to stop asserting the interrupt
before returning.  If it doesn't, we will immediately take the interrupt
again when the handler returns and the generic code unmasks the interrupt.

The driver doesn't know whether its interrupt is edge- or level-triggered,
so it must clear its interrupt source directly in its interrupt handler.

Previously we cleared the DPC interrupt status in the bottom half, i.e., in
deferred work, which can cause an interrupt storm if the DPC interrupt
happens to be level-triggered, e.g., if we're using INTx instead of MSI.

Clear the DPC interrupt status bit in the interrupt handler, not in the
deferred work.

Signed-off-by: Oza Pawandeep <poza@codeaurora.org>
[bhelgaas: changelog]
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>
Reviewed-by: Keith Busch <keith.busch@intel.com>