staging: comedi: amplc_dio200: add helpers to get board layout

Add inline helper function `dio200_board_layout(board)` to get a pointer
to the board layout data for a board.  Add inline helper function
`dio200_dev_layout(dev)` to get a pointer to the board layout data for a
comedi device (this function is currently unused but will be used by a
later change).

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>