staging: comedi: dt2801: remove 'boardtype' macro

The 'boardtype' macro relies on a local variable having a specific
name and yields a struct derived from that local variable.

Replace the macro with local variables and use the comedi_board()
helper to get the struct as a pointer. Use pointer access when
using the variable.

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