- Ensure ports are detected in the right order (also fixes crash in
  interrupt handler).
- Make use of uart api port fields: port, line, irq, fifosize, mapbase. 
- Remove some variable fifo size logic, driver always uses 32 byte fifos. 
- Unmask XPR interrupt in transmit_chars() (per 2.4).
- Unmask XPR interrupt early in sunsab_start_tx(), as the user may have 
  been put to sleep by the uart api with XPR masked, and will otherwise 
  not be able to unmask XPR again.
- Do not enable RFDF, as the code is no longer checking for or 
  even handling rx status bytes.
- Make sunsab_type() return more useful info.
- Increment instance_p in sab_attach_callback().
- Remove self-reporting, as the uart api now reports the ports.
- Handle sunserial_current_minor increment/decrement properly.