USB pl2303 driver

This is a patch which avoids resetting the termios settings to default
values (9600 Baud etc.) in each call to pl2303_open (). It does this
only on the first call to pl2303_set_termios. After that it sets the
termios to the last stored values.

 This way commands like
 stty -F /dev/ttyUSB0 115200
 work the same way as with other serial ttys.