USB: serial: opticon: add serial line ioctls

This lets userspace determine what the state of the RTS line is, which
is what is needed to properly handle data flow for this device (it
raises RTS when there is data to be sent from it.)

Cc: Kees Stoop <kees.stoop@opticon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>