com driver headaches

[Jim Rees <rees@umich.edu>]

listbin@quix.ch and I have been running some tests, and I have come to some
preliminary conclusions about the state of com driver brokenness in 2.2 (and
2.1?) i386.

The com driver used with a 550 uart receives fine but drops characters on
transmit when it uses hardware handshaking.  The pccom driver seems to work
ok.

The pccom driver used with a 450 uart (no fifo) drops characters on receive.
The com driver works better.

This is awkward for me because one of my machines has one 450 uart and one
550 uart.