USB: serial: cp210x: fix modem-control handling

The vendor request used to set the flow-control settings also sets the
state of the modem-control lines.

Add state variables to keep track of the modem-control lines to avoid
always asserting the lines whenever the flow-control settings are
updated.

This specifically also avoids asserting DTR/RTS when opening a port with
the line speed set to B0.

Signed-off-by: Johan Hovold <johan@kernel.org>