USB: pl2303: fix DTR/RTS being raised on baud rate change

DTR/RTS should only be raised when changing baudrate from B0 and not on
any baud rate change (> B0).

Reported-by: Søren Holm <sgh@sgh.dk>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>