maple: fix wrong return value of maple_bus_init().

If KMEM_CACHE or maple_alloc_dev failed, the maple_bus_init() will return 0
rather than error, because the retval is not changed after KMEM_CACHE or
maple_alloc_dev failed.

Fixes: 17be2d2b ("sh: Add maple bus support for the SEGA Dreamcast.")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Lu Wei <luwei32@huawei.com>
Acked-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: Rich Felker <dalias@libc.org>