DMA: Deduplicate descriptor operations (#2219)

* Deduplicate descriptor operations

* Reuse constructor

* Reuse linked list construction logic

* Extract count calculations into DescriptorSet

* Reuse more code in DescriptorChain

* Refactor DescriptorChain

* Move DescriptorSet next to DescriptorChain

* Restore comment

* Merge fns

* Implement error conversion

* Further cleanup

* Rearrange some code

* Add some docs

* Clippy

* Restore macro behaviour when requesting 0 descriptors

* Restore the buffer directly, instead of tracking where we read from

* Move validation back into the buffers

* Revert use in dma_tx_buffer

* Rename iterator methods

* Outline eof logic

* Don't set up descriptor flags before time, rename methods

* Hide DescriptorSet

* Remove block size from DescriptorSet

* Move comment

* Fix renamed references

* Address review comments

* Simplify received_data fns