USB: serial: ch341: fix modem-control and B0 handling
The modem-control signals are managed by the tty-layer during open and
should not be asserted prematurely when set_termios is called from
driver open.
Also make sure that the signals are asserted only when changing speed
from B0.
Fixes: 664d5df92e88 ("USB: usb-serial ch341: support for DTR/RTS/CTS") Cc: stable <stable@vger.kernel.org> Signed-off-by: Johan Hovold <johan@kernel.org>