load_elf() gives negative return in case of error, not zero.

Fixes: 10e3f30f ("hw/mips/boston: Allow loading elf kernel and dtb")
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20211130211729.7116-3-jiaxun.yang@flygoat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>