* Uses Counter/Timer dynamicly for rate generation; You can
have both 19200 and 38400 on one duart, and 57600 on another!
* Improved scninfo program to dump driver internal information.
implemented counters for parity err, framing err, break
* Delays changes until transmitter idle to avoid "glitches"
* Cleaned up scn_softc member names
* Untested support for sc26c92 (I don't yet have any to test with);
+ Will use alternate rate tables (include 115.2kbps and 230.4kbps)
+ Will raise FIFO rx interrupt threshold and watchdog timer
at higher rates.
+ Unknown (to me) how fast pc532 MC145406 driver chips can go
NOTE: It seems like it may be a BAD idea to use a sc26c92 as
a console port for now; the PROM doesn't know to reset the
chip back to scn2692 mode!!
* Define SCN_RING_HIWAT and add sc_rbhiwat to scn_softc. When the
ringbuffer gets full up to this mark, rts gets asserted (Ian Dall).
* Add sc_rx_blocked to scn_softc. This is used to tell scnsoft that
rxinterrupts are blocked (Ian Dall).
