staging: comedi: usbdux: bug fix for accessing 'ao_chanlist' in private data

In usbdux_ao_cmd(), the channels for the command are transfered from the
cmd->chanlist and stored in the private data 'ao_chanlist'. The channel
numbers are bit-shifted when stored so that they become the "command"
that is transfered to the device. The channel to command conversion
results in the 'ao_chanlist' having these values for the channels:

  channel 0 -> ao_chanlist = 0x00
  channel 1 -> ao_chanlist = 0x40
  channel 2 -> ao_chanlist = 0x80
  channel 3 -> ao_chanlist = 0xc0

The problem is, the usbduxsub_ao_isoc_irq() function uses the 'chan' value
from 'ao_chanlist' to access the 'ao_readback' array in the private data.
So instead of accessing the array as 0, 1, 2, 3, it accesses it as 0x00,
0x40, 0x80, 0xc0.

Fix this by storing the raw channel number in 'ao_chanlist' and doing the
bit-shift when creating the command.

Fixes: a998a3db

 "staging: comedi: usbdux: cleanup the private data 'outBuffer'"
Cc: stable <stable@vger.kernel.org> # 3.12
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Acked-by: Bernd Porr <mail@berndporr.me.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>