mips_malta: cap BIOS endian swap length at 0x3e0000 bytes

This preserves the final sector of the pflash which is used by YAMON to
hold environment variables. If the endianness of the environment data
is swapped then YAMON will fail to load environment variables from
pflash.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

hw/mips/mips_malta.c

@@ -978,7 +978,7 @@ void mips_malta_init(QEMUMachineInitArgs *args)
             if (!addr) {
                 addr = memory_region_get_ram_ptr(bios);
             }
-            end = (void *)addr + bios_size;
+            end = (void *)addr + MIN(bios_size, 0x3e0000);
             while (addr < end) {
                 bswap32s(addr);
                 addr++;