staging: comedi: das1800: remove 'thisboard' macro

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

Replace the macro with local variables and use the comedi_board()
helper to get the pointer.

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>