staging: comedi: usbduxsigma: tidy up usbdux_dio_insn_bits()

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

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>