staging: comedi: usbduxfast: only chanlist of 1, 2, 3, or 16 are supported

The analog input (*do_cmd) only supports channel lists of 1, 2, 3, or 16
channels. Add a check for this to usbduxfast_ai_check_chanlist() and remove
the check from usbduxfast_ai_cmd().

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>