blk-mq: Remove unnecessary local variable

Remove unnecessary local variable 'ret' in blk_mq_dispatch_hctx_list().

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>