USB: usb-serial ch341: support for DTR/RTS/CTS

Fixup of Werner Cornelius patch to the ch341 USB-serial driver, which adds:
- support all baudrates, not just a hard-coded set
- support for controlling DTR, RTS and CTS

Features still missing:
- character length other than 8 bits
- parity settings
- break control

I adapted his patch for the new usb_serial API introduced in 2.6.25-git8 by
Alan Cox on 22 July 2008. Non-compliance to the new API was a reason for
refusing a similar patch from Tollef Fog Heen.

Usage example by Tollef Fog Heen :
        TEMPer USB thermometer <http://err.no/src/TEMPer.c>
Signed-off-by: Werner Cornelius <Werner.Cornelius@cornelius-consult.de>
Signed-off-by: Boris Hajduk <boris@hajduk.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>