PCI/MSI: Make pci_msi_domain_write_msg() static

There is no point to have this function public as it is set by the PCI core
anyway when a PCI/MSI irqdomain is created.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>	# PCI
Link: https://lore.kernel.org/r/20211206210224.157070464@linutronix.de