USB: serial: cp210x: fix hardware flow-control disable

A bug in the CRTSCTS handling caused RTS to alternate between

CRTSCTS=0 => "RTS is transmit active signal" and
CRTSCTS=1 => "RTS is used for receive flow control"

instead of

CRTSCTS=0 => "RTS is statically active" and
CRTSCTS=1 => "RTS is used for receive flow control"

This only happened after first having enabled CRTSCTS.

Signed-off-by: Konstantin Shkolnyy <konstantin.shkolnyy@gmail.com>
Fixes: 39a66b8d ("[PATCH] USB: CP2101 Add support for flow control")
Cc: stable <stable@vger.kernel.org>
[johan: reword commit message ]
Signed-off-by: Johan Hovold <johan@kernel.org>