USB: serial: pl2303: add support for tx xon/xoff flow control

Support hardware-level Xon/Xoff flow control in transmit direction with
pl2303.

I only know how to get the hardware to do IXON/!IXANY with ^S/^Q as control
characters, so I preserve the old behaviour for all other cases.

Signed-off-by: Florian Zumbiehl <florz@florz.de>
[ johan: rewrite logic using pl2303_termios_change() helper ]
Signed-off-by: Johan Hovold <johan@kernel.org>