PCI/MSI: Use __free() for affinity masks

Let cleanup handle the freeing of the affinity mask. That prepares for
switching the MSI descriptor locking to a guard().

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/all/20250319105506.444764312@linutronix.de