PCI: rcar-host: Pass proper IRQ domain to generic_handle_domain_irq()

Starting with commit dd26c1a2 ("PCI: rcar-host: Switch to
msi_create_parent_irq_domain()"), the MSI parent IRQ domain is NULL because
the object of type struct irq_domain_info passed to:

msi_create_parent_irq_domain() ->
  irq_domain_instantiate()() ->
    __irq_domain_instantiate()

has no reference to the parent IRQ domain. Using msi->domain->parent as an
argument for generic_handle_domain_irq() leads to below error:

	"Unable to handle kernel NULL pointer dereference at virtual address"

This error was identified while switching the upcoming RZ/G3S PCIe host
controller driver to msi_create_parent_irq_domain() (which was using a
similar pattern to handle MSIs (see link section)), but it was not tested
on hardware using the pcie-rcar-host controller driver due to lack of
hardware.

Fixes: dd26c1a2 ("PCI: rcar-host: Switch to msi_create_parent_irq_domain()")
Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
[mani: reworded subject and description]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Nam Cao <namcao@linutronix.de>
Link: https://lore.kernel.org/all/20250704161410.3931884-6-claudiu.beznea.uj@bp.renesas.com
Link: https://patch.msgid.link/20250809144447.3939284-1-claudiu.beznea.uj@bp.renesas.com