altos/stm: Note that ao_i2c_recv_dma_isr isn't actually used

This function resets the i2c transceiver after DMA completes, but it
hasn't ever been hooked up. Some testing would be a good idea. For
now, just add a comment and mark the function as referenced to avoid a
compiler warning.

Signed-off-by: Keith Packard <keithp@keithp.com>