bl_gen_write_ulong uses sd for both 32 and 64 bit CPU,
while sd is illegal on 32 bit CPUs.

Replace sd with sw on 32bit CPUs.

Fixes: 3ebbf861 ("hw/mips: Add a bootloader helper")
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20211130211729.7116-2-jiaxun.yang@flygoat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>