staging: comedi: 8255: document callback parameters better

Several Comedi driver modules call `subdev_8255_init()` or
`subdev_8255_mm_init()` to set up a digital I/O subdevice based on the
"8255" chip.  One of the parameters to these functions is an optional
pointer to an I/O callback function to perform the actual register
accesses (an internal default callback function is used if NULL).

The kerneldoc for `subdev_8255_init()` and `subdev_8255_mm_init()`
describe the prototype of the optional I/O callback function
incorrectly (my fault), adding a non-existent parameter of type `struct
comedi_subdevice *`.  Fix the kerneldoc.  Also add parameter names to
the callback function pointer type wherever it occurs to make the usage
clearer.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>