prep: Use ISA m48t59

This simplifies the code later when the i8259 moves to the i82378
PCI->ISA bridge and happens to fix a SysBus m48t59 io_base issue
introduced by commit 0fb56ffc5e (m48t59:
drop obsolete address base arithmetic). Suggested by Hervé and Jan.

Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Cc: Hervé Poussineau <hpoussin@reactos.org>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Blue Swirl <blauwirbel@gmail.com>

hw/ppc_prep.c

@@ -707,7 +707,7 @@ static void ppc_prep_init (ram_addr_t ram_size,
         usb_ohci_init_pci(pci_bus, -1);
     }
 
-    m48t59 = m48t59_init(i8259[8], 0, 0x0074, NVRAM_SIZE, 59);
+    m48t59 = m48t59_init_isa(isa_bus, 0x0074, NVRAM_SIZE, 59);
     if (m48t59 == NULL)
         return;
     sysctrl->nvram = m48t59;