staging: comedi: usbdux: remove 'ao_chanlist' from private data

This member of the private data is just a copy of the channels from
the cmd->chanlist. Remove the member and just use the cmd->chanlist
directly.

Remove the unneeded 'len' local variable in usbdux_ao_isoc_irq().
This is just the cmd->chanlist_len.

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>