staging: comedi: usbduxsigma: tidy up usbdux_pwm_start()

Rename the function so it has namespace associated with the driver.
Rename the local variable used for the private data pointer.

Remove the unnecessary cast of the pwm delay.

Use memset to initialize the buffer instead of the for loop.

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>