hw/ssi: Check for duplicate CS indexes

This to avoid indexes conflicts on the same SSI bus. Adapt machines
using multiple devices on the same bus to avoid breakage.

Cc: "Edgar E. Iglesias" <edgar.iglesias@gmail.com>
Cc: Alistair Francis <alistair@alistair23.me>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

hw/microblaze/petalogix_ml605_mmu.c

@@ -192,6 +192,7 @@ petalogix_ml605_init(MachineState *machine)
                                         blk_by_legacy_dinfo(dinfo),
                                         &error_fatal);
             }
+            qdev_prop_set_uint8(dev, "cs", i);
             qdev_realize_and_unref(dev, BUS(spi), &error_fatal);
 
             cs_line = qdev_get_gpio_in_named(dev, SSI_GPIO_CS, 0);