hw/ppc/ppc4xx_pci: Replace magic value by the PCI_NUM_PINS definition

Replace the magic '4' value by the PCI_NUM_PINS definition.

Suggested-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200910072325.439344-3-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>