prep: Make System I/O port 0092 read/write

Port 0x0092 is documented as read/write, so for now return the
endianness state instead of hardcoded 0x00.
Signed-off-by: NJulio Guerra <guerr@julio.in>
[AF: Extracted from larger port 0092 patch]
Signed-off-by: NAndreas Färber <andreas.faerber@web.de>

hw/ppc/prep.c

@@ -269,7 +269,7 @@ static uint32_t PREP_io_800_readb (void *opaque, uint32_t addr)
     switch (addr) {
     case 0x0092:
         /* Special port 92 */
-        retval = 0x00;
+        retval = sysctrl->endian << 1;
         break;
     case 0x0800:
         /* Motorola CPU configuration register */