staging: comedi: cb_pcimdas: remove thisboard and devpriv macros

These macros rely on a local variable having a specific name.
Remove them and use the comedi_board() helper to get the thisboard
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>