staging: comedi: pcl730: tidy up pcl730_di_insn()

For aesthetic reasons, rename the function.

Use a local variable for the 'reg' offset that is stored in the
comedi_subdevice 'private' pointer to minimize the number of 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>