midi host: raise default RX FIFO above EP size, document drain requirement

Follow-up to #3239. tuh_midi_stream_read terminates on cable-number
transitions, leaving residue in the FIFO. With the default RX FIFO
sized equal to one bulk packet, the next bulk IN transfer fails to
queue and the driver silently stops receiving. Raising the default
to 2x bulk gives single-call apps a full packet of headroom and
documents the drain-loop expectation.

Reproduced with Akai LPD8 mk1 (VID 09E8 PID 0075) on STM32H753 DWC2
host; fixed with this patch. See #3613 for full repro + captures.

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