m68k: set proper major_num when specifying module param major_num

When calling register_blkdev() with specified major
device number, the return code is 0 on success.
So it seems not correct direct assign return code to
variable major_num in this case.

Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>