staging: comedi: das16: cleanup analog output subdevice

All the boards supported by this driver that have analog outputs use
the same function to handle the (*insn_write) for the subdevice. They
all also have 12-bit resolution (maxdata = 0x0fff).

Remove the 'ao' and 'ao_nbits' members from the boardinfo and replace
them with a simple bit-field flag 'has_ao'.

Tidy up the subdevice init a bit as well as the (*insn_write) function.

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>