powerpc/pseries/msi: Add an empty irq_write_msi_msg() handler

The IPR drivers tests for MSI support at probe time with MSI vector 0
and when done, frees the IRQ with free_irq(). This test was introduced
by 95fecd90 ("ipr: add test for MSI interrupt support") as an
improvement of commit 5a9ef25b ("[SCSI] ipr: add MSI support")
because a boot failure was reported on a Bimini PowerPC system:

  https://lore.kernel.org/r/1242926159.3007.5.camel@localhost.localdomain



It was finally decided to remove MSI support on Bimini systems in
6eb0ac03 ("powerpc/maple: Add a quirk to disable MSI for IPR on
Bimini").

Linux 5.15-rc1 added MSI domain support to the pseries machine and
when free_irq is called() in the driver, msi_domain_deactivate() also
is. This resets the MSI table entry of the associate vector by calling
__pci_write_msi_msg() with an empty message and breaks any further
activation of the same vector. In the case of the IPR driver, it
breaks the initialization sequence of the IOA.

Introduce an empty irq_write_msi_msg() handler in the MSI domain of
the pseries machine to avoid clearing the MSI vector entry. Updating
the entry is not strictly necessary since it is initialized by the
underlying hypervisor, PowerVM or QEMU/KVM.

Fixes: a5f3d2c1 ("powerpc/pseries/pci: Add MSI domains")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reported-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Tested-by: Mahesh Salgaonkar <mahesh@linux.ibm.com>
[mpe: Tweak comment wording and formatting slightly]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210930102535.1047230-1-clg@kaod.org