PCI: designware: Setup and clear exactly one MSI at a time

The setup_irq function is supposed to set up exactly one MSI IRQ.  Multiple
IRQ setup is handled differently, to respect the choices made by the upper
layers.

Also only clear one MSI IRQ at a time; the PCI core will call into this
function multiple times if it has to tear down more than one MSI IRQ.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Pratyush Anand <pratyush.anand@st.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>