PCI: xgene-msi: Make per-CPU interrupt setup robust

PCI: xgene-msi: Make per-CPU interrupt setup robust

The way the per-CPU interrupts are dealt with in the XGene MSI
driver isn't great:

- the affinity is set after the interrupt is enabled

- nothing prevents userspace from moving the interrupt around

- the affinity setting code pointlessly allocates memory

- the driver checks for conditions that cannot possibly happen

Address all of this in one go, resulting in slightly simpler setup
code.

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/20250708173404.1278635-6-maz@kernel.org