staging: comedi: usbduxsigma: use usbdux_pwm_stop() to force unlink urb

In tidy_up(), instead of duplicating the code in usbdux_pwm_stop() to
unlink the pwm urb, just use usbdux_pwm_stop(). Since that function calls
usb_kill_urb() and clears the 'pwm_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>