powerpc/pseries/msi: Use msi_domain_ops:: Msi_post_free()

Use the new msi_post_free() callback which is invoked after the interrupts
have been freed to tell the hypervisor about the shutdown.

This allows to remove the exposure of __msi_domain_free_irqs().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ashok Raj <ashok.raj@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20221111122014.120489922@linutronix.de