Fix receiving ACKs: enable pipe 0, disable pipe 1 in Tx.
Don't rely on TX_FULL when checking for Tx result.
Make the ring buffer size fit 8 bits.

Otherwise .start overflows and old data gets resent from the start of
the buffer.  We could make .start and .len 16-bit, but .len accesses
would not be atomic anymore so it might not be worth it.