staging: comedi: me4000: refactor 'ao_fifo' boardinfo

This member of the boardinfo is always '4' for the boards that have an
analog output FIFO. Covert it to a bit-field, 'has_ao_fifo', to save a
bit of space.

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