powerpc/powernv/pci: Rework pnv_opal_pci_msi_eoi()

powerpc/powernv/pci: Rework pnv_opal_pci_msi_eoi()

pnv_opal_pci_msi_eoi() is called from KVM to EOI passthrough interrupts
when in real mode. Adding MSI domain broke the hack using the
'ioda.irq_chip' field to deduce the owning PHB. Fix that by using the
IRQ chip data in the MSI domain.

The 'ioda.irq_chip' field is now unused and could be removed from the
pnv_phb struct.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210701132750.1475580-30-clg@kaod.org