null_blk: remove unused variable warning on !CONFIG_BLK_DEV_ZONED

If BLK_DEV_ZONED isn't set, 'ret' isn't used. This makes gcc complain,
rightfully. Move ret where it is used.

Fixes: 979d5447

 ("null_blk: cleanup null_gendisk_register")
Signed-off-by: Jens Axboe <axboe@kernel.dk>