staging: comedi: 8255.c: document subdev_8255_init() and _mm_init()

Add kerneldoc comments for the subdev_8255_init() and
subdev_8255_mm_init() functions and remove the other comment about how
to use this module in a comedi driver.

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>