hw/ppc/ppc4xx_pci: Replace pointless warning by assert()

hw/ppc/ppc4xx_pci: Replace pointless warning by assert()

We call pci_register_root_bus() to register 4 IRQs with the
ppc4xx_pci_set_irq() handler. As it can only be called with
values in the [0-4[ range, replace the pointless warning by
an assert().

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20200901104043.91383-5-f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>