This implements the bus transmit/receive/sigchg/ipend methods with
a polled interrupt handler (ipend) rather than enabling hardware
interrupts.
The FIFO is faked at 16 bytes deep for now, just so the transmit
IO side doesn't suck too bad (the callout frequency limits how quickly
IO is flushed to the sender, rather than scheduling the callout more
frequently whilst there's active TX. But I digress.)
Tested:
* Atheros AP121 (AR9330) reference board, booting to multi-user interactive
mode.
