staging: comedi: usbduxsigma: tidy up usbduxsigma_pwm_config()

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

Remove some unnecessary comments and tidy up the comment about the
parameters for the INSN_CONFIG_PWM_SET_H_BRIDGE instruction.

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>