staging: comedi: usbdux: fix usbdux_pwm_cancel()

Add the missing down/up of the semaphore to prevent other commands
from being issued to the usb device while the pwn is being stopped.

Rename the local variable used for the private data pointer to the
comedi "norm".

Make sure to check that usbdux_pwm_stop() was successful before
sending command to the usb device to stop the pwm.

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>