fix: align DMA buffers to 32bytes on H7

[3djc]

Align DMA buffers to 32bytes on H7

Fixes #6509

Thanks Radiomaster for spotting that one.

[richardclli]

@3djc Did you read my comment in #6509 ?

[3djc]

I did, but my goal here is not to optimize dma buffer by splitting read and write, but simply fixing a wrong alignment we are using today.

Your link actually highlight the need for this change: "Please note that in case of reception there can be problem if rx_buffer is not aligned to the size of cache-line (32-bytes), because during the invalidate operation another data sharing the same cache-line(s) with rx_buffer can be lost."

[pfeerick]

This was also in #6484 so not merging.