usb:usbserial:opticon: fix race between unthrottle and completion handler

opticon_unthrottle() mustn't resubmit the URB unconditionally
as the URB may still be running.
Signed-off-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>