rsxx: remove redundant NULL check

Fix below warnings reported by coccicheck:
./drivers/block/rsxx/dma.c:948:3-8: WARNING: NULL check
before some freeing functions is not needed.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <abaci-bugfix@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>