staging: comedi: me4000: refactor 'ao_nchan' boardinfo

For the boards that have analog output capability, there are always
4 analog output channels. Convert the 'ao_nchan' member of the boardinfo
into a bit-field, 'has_ao', to save a bit of space and set the analog
output subdevice 'n_chan' to 4 when supported.

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>