PCI/MSI: Use msi_mask_irq() in pci_msi_shutdown()

No point in using the raw write function from shutdown. Preparatory change
to introduce proper serialization for the msi_desc::masked cache.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20210729222542.674391354@linutronix.de