staging: comedi: usbduxsigma: tidy up usbdux_pwm_period()

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

Modify the fx2delay calculation to remove the floating point values.

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>