staging: comedi: adl_pci6208: fix the ao_readback variable size

staging: comedi: adl_pci6208: fix the ao_readback variable size

The 'ao_readback' variable in the private data struct is used to
cache the last value written to the analog output DAC channels.
The PCI-6208 has 8 analog output channels but the variable only
allowed space to cache 2 values.

Since the PCI-6216 board could be supported by this driver and
it has 16 analog outputs, create a define for the maximum number
of channels and use that to set the size of 'ao_readback'.

For now, set the max to 8 to save space since the PCI-6216 is
not currently supported.

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