staging: comedi: rtd520: cleanup rtd_dio_insn_config()

Add a local variable to make this function a bit cleaner and
remove the unnecessary comments.

The comedi core expects this function to return the number of
data parameters used. Change the return from '1' to 'insn->n'
to make this more apparent.
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>