staging: comedi: cb_pcidas: Allow 2-channel commands for AO subdevice

The "cb_pcidas" driver supports asynchronous commands on the analog
output (AO) subdevice for those boards that have an AO FIFO.  The code
(in `cb_pcidas_ao_check_chanlist()` and `cb_pcidas_ao_cmd()`) to
validate and set up the command supports output to a single channel or
to two channels simultaneously (the boards have two AO channels).
However, the code in `cb_pcidas_auto_attach()` that initializes the
subdevices neglects to initialize the AO subdevice's `len_chanlist`
member, leaving it set to 0, but the Comedi core will "correct" it to 1
if the driver neglected to set it.  This limits commands to use a single
channel (either channel 0 or 1), but the limit should be two channels.
Set the AO subdevice's `len_chanlist` member to be the same value as the
`n_chan` member, which will be 2.

Cc: <stable@vger.kernel.org>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20201021122142.81628-1-abbotti@mev.co.ukSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/cb_pcidas.c

@@ -1342,6 +1342,7 @@ static int cb_pcidas_auto_attach(struct comedi_device *dev,
 		if (dev->irq && board->has_ao_fifo) {
 			dev->write_subdev = s;
 			s->subdev_flags	|= SDF_CMD_WRITE;
+			s->len_chanlist	= s->n_chan;
 			s->do_cmdtest	= cb_pcidas_ao_cmdtest;
 			s->do_cmd	= cb_pcidas_ao_cmd;
 			s->cancel	= cb_pcidas_ao_cancel;