staging: comedi: dt282x: factor board initialization out of (*attach)

For aesthetics, factor the code that initializes the then checks to see
if it is actually a DT2821 series board out of the (*attach).

For clarity, remove the MASK and VAL defines used to check the board and
jsut open code the values.

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>