block: ataflop: Fix warning comparing pointer to 0

Fix the following coccicheck warning:

./drivers/block/ataflop.c:1464:20-21: WARNING comparing pointer to 0.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Link: https://lore.kernel.org/r/1635501029-81391-1-git-send-email-jiapeng.chong@linux.alibaba.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>