staging: comedi: usbduxsigma: 'dac_commands' does not need to be kzalloc()'d

The 'dac_commands' buffer is used to pass the ao channel list from
usbduxsigma_ao_cmd() to the urb callback, usbduxsigma_ao_urb_complete().
This buffer does not need to be allocated.

Change it into a simple array of the correct size. Rename the variable
to 'ao_chanlist' to clarify what it actually is.

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>