PCI: iproc: Fix incorrect MSI address alignment

In the code to handle PAXB v2 based MSI steering, the logic aligns the MSI
register address to the size of supported inbound mapping range.  This is
incorrect since it rounds "up" the starting address to the next aligned
address, but what we want is the starting address to be rounded "down" to
the aligned address.

This patch fixes the issue and allows MSI writes to be properly steered to
the GIC.

Fixes: 4b073155fbd3 ("PCI: iproc: Add support for the next-gen PAXB controller")
Signed-off-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>