USB: serial: cp210x: add support for B0 hangup

A request to set the line speed to B0 is used to hang up a modem
connection by deasserting the modem control lines.

Note that there is no need reconfigure the line speed in hardware when
B0 is requested (even if some drivers do set it to an arbitrary value
for implementation or protocol reasons).

Reviewed-by: Alex Henrie <alexh@vpitech.com>
Signed-off-by: Johan Hovold <johan@kernel.org>