feat(spi): Unlock DMA transfers for SpiBus::transfer_in_place

[jbeaurivage]

Fixes unsoundness issues caused by mishandled memory barriers in the dmac module.

These were causing observable undefined behaviour in the SpiBus::transfer_in_place implementation when trying to use DMA. The implementation was therefore defaulted to using word-by-word transfers in #772.

Now that the bug is fixed, we also unlock DMA in-place transfers for SPI. I've tested the changes and can confirm that there is no longer exhibited unsoundness.

This PR also brings minor refactors to the dmac module, and fixes warnings when compiling with the dma Cargo feature turned off.

Checklist

• All new or modified code is well documented, especially public items
• No new warnings or clippy suggestions have been introduced - CI will deny clippy warnings by default! You may #[allow] certain lints where reasonable, but ideally justify those with a short comment.