staging: comedi: addi_common: replace comedi_board() calls

The `comedi_board(dev)` inline function calls just return
`dev->board_ptr`.  Expand the inline function calls.

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