staging: comedi: ni_mio_common: remove some unnecessary boardinfo access

Currently ni_m_series_ao_config_chanlist() gets the boardinfo pointer in
order to get the number of analog output channels. Get this from the
comedi_subdevice instead.

The ni_old_ao_config_chanlist() function also gets the boardinfo pointer
to get the analog output resolution, 'aobits', in order to calculate the
value needed to convert samples from unsigned to two's complement for
bipolar ranges. Use the comedi_subdevice 'maxdata' instead.

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>