staging: comedi: usbduxsigma: use usbdux_ao_stop() to force unlink all urbs

In tidy_up(), instead of duplicating the code in usbdux_ao_stop() to unlink
all the output urbs, just use usbdux_ao_stop(). Since that function calls
usb_kill_urb() for all the urbs and clears the 'ao_cmd_running' flag, we
can also remove the redundant code in tidy_up().

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>