prep: Kill get_system_io() usage

While ISA address space in prep machine is currently the one returned
by get_system_io(), this depends of the implementation of i82378/raven
devices, and this may not be the case forever.

Use the right ISA address space when adding some more ports to it.
We can use whatever ISA device on the right ISA bus, as all ISA devices
on the same ISA bus share the same ISA address space.
Signed-off-by: NHervé Poussineau <hpoussin@reactos.org>
Signed-off-by: NAndreas Färber <andreas.faerber@web.de>

hw/ppc/prep.c

@@ -656,7 +656,7 @@ static void ppc_prep_init(QEMUMachineInitArgs *args)
     sysctrl->reset_irq = cpu->env.irq_inputs[PPC6xx_INPUT_HRESET];
 
     portio_list_init(port_list, NULL, prep_portio_list, sysctrl, "prep");
-    portio_list_add(port_list, get_system_io(), 0x0);
+    portio_list_add(port_list, isa_address_space_io(isa), 0x0);
 
     /* PowerPC control and status register group */
 #if 0