staging: comedi: plx9080.h: tidy up some comments

Use the usual block comment style and remove some excess spaces from
single-line comments.  Remove some obvious stuff about register offsets
being zero relative.  Remove some text about the original history of the
file, as it bears hardly any resemblance to it now.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>