Factor out the PCI MSI message setup from the single MSI setup function.
This will be reused by the multivector MSI setup.

No functional change yet.
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Pratyush Anand <pratyush.anand@gmail.com>