staging: comedi: usbduxsigma: tidy up usbdux_dio_insn_config()

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

Remove the unnecessary comments and clean up the multi-line comment
at the end.

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>