staging: comedi: pcl730: tidy up pcl730_do_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.
Also, add local variables for the 'mask' and 'bits'.

The outputs only need to be updated if the 'mask' indicates that the
'bits' are changing. Move the update code into the main 'if (mask)'
block.

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>