PCI: designware: Reject MSI-X IRQs

The DesignWare PCIe MSI hardware does not support MSI-X IRQs.  Setting
those up failed as a side effect of a bug which was fixed by 91f8ae82
("PCI: designware: Setup and clear exactly one MSI at a time").

Now that this bug is fixed, MSI-X IRQs need to be rejected explicitly;
otherwise devices trying to use them may end up with incorrectly working
interrupts.

Fixes: 91f8ae82 ("PCI: designware: Setup and clear exactly one MSI at a time")
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>
CC: stable@vger.kernel.org	# v3.18+