ppc/pnv: Fix number of registers in the PCIe controller on POWER9

The spec defines 3 registers, even though only index 0 and 2 are valid
on POWER9. The same model is used on POWER10. Register 1 is defined
there but we currently don't use it in skiboot. So we can keep
reporting an error on write.

Reported by Coverity (CID 1487176).

Fixes: 4f9924c4 ("ppc/pnv: Add models for POWER9 PHB4 PCIe Host bridge")
Suggested-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220401091925.770803-1-fbarrat@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

include/hw/pci-host/pnv_phb4.h

@@ -180,7 +180,7 @@ struct PnvPhb4PecState {
     MemoryRegion nest_regs_mr;
 
     /* PCI registers, excluding per-stack */
-#define PHB4_PEC_PCI_REGS_COUNT     0x2
+#define PHB4_PEC_PCI_REGS_COUNT     0x3
     uint64_t pci_regs[PHB4_PEC_PCI_REGS_COUNT];
     MemoryRegion pci_regs_mr;