staging: comedi: usbduxsigma: generalize the ai/ao usb_kill_urb()

Generalize a helper function to replace for() loops in usbduxsigma_{ai,ao}_stop()
that call usb_kill_urb() to unlink all the urbs.

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