USB: serial: ch341: add hardware flow control RTS/CTS

This adds support for enabling and disabling RTS/CTS hardware flow
control.

Tested using CH341A and CH340E.

Fixes part of the following bug report:
Link: https://bugzilla.kernel.org/show_bug.cgi?id=197109



Signed-off-by: Lode Willems <me@lodewillems.com>
[ johan: prepare index argument once, drop casts ]
Signed-off-by: Johan Hovold <johan@kernel.org>