ports/stm: enlarge audioio DMA buffer to 2048 samples

256-sample halves left only ~5 ms of headroom before underrun at
44.1 kHz. USB enumeration, VFS sync and other main-loop work can
exceed that, producing audible glitches. Bumping to 1024-sample
halves (21 ms at 48 kHz) gives comfortable margin while still keeping
total buffer memory at 4 KB per channel.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>