USB: RTS/CTS handshaking support, DTR fixes for MCT U232 serial adapter

Improvements and fixes to the MCT U232 USB/serial interface driver.
Implement RTS/CTS hardware flow control.  Implement HUPCL.  Bring
handling of DTR and RTS into conformance with other Linux serial
port drivers - assert both signals when opening device, even if
"crtscts" is not currently selected.

Signed-off-by: Dave Platt <dplatt@radagast.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>