staging: comedi: dt282x: tidy up the register map and bit defines

For aesthetics, rename the register map defines so that the bit defines
are associated with the registers. Also, convert all the bit defines into
bit-shifts to make them clear.

Remove all the verbose comments. All the registers and bits are named
according to the User Manual for the DT2821 Series, Document UM-05073-G.
That manual can be checked if necessary for the verbose information.
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>