PCI: xgene-msi: Get rid of intermediate tracking structure

PCI: xgene-msi: Get rid of intermediate tracking structure

The xgene-msi driver uses an odd construct in the form of an
intermediate tracking structure, evidently designed to deal with
multiple instances of the MSI widget. However, the existing HW
only has one set, and it is obvious that there won't be new HW
coming down that particular line.

Simplify the driver by using a bit of pointer arithmetic instead,
directly tracking the interrupt and avoiding extra memory allocation.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20250708173404.1278635-9-maz@kernel.org