staging: comedi: remove comedi_board()

All calls to the inline function `comedi_board()` in "comedidev.h" have
been removed, so remove the function.

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>