staging: comedi: daqboard2000: support 4 AO channels

The driver supports DaqBoard/2000 and DaqBoard/2001. DaqBoard/2000 has 2
AO channels, but DaqBoard/2001 has 4 AO channels.  The driver currently
only supports 2 AO channels, but supporting 4 channels is just a case of
setting the `n_chan` member of the COMEDI subdevice to 4 instead of 2.

Add a new boolean flag member `has_2_ao` to `struct db2k_boardtype` to
be set to `true` if the board only has 2 AO channels.  Set this to
`true` in the element of `db2k_boardtypes[]` that corresponds to the
DaqBoard/2000.  Use it in `db2k_auto_attach()` to initialize the number
of AO channels to 2 or 4, as appropriate.

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