staging: comedi: cb_das16_cs: remove incomplete async command support

This driver has a (*do_cmdtest) function for analog input async command
support but the (*do_cmd) function just returns -EINVAL.

Remove the incomplete async command support.

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>