staging: comedi: cb_das16_cs: refactor 'n_ao_chans' boardinfo

Only one of the boards supported by this driver has analog outputs.
For aesthetics, change the 'n_ao_chans' member of the boardinfo into
a bit-field flag 'has_ao'.

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>