staging: comedi: usbduxsigma: check for invalid 'ao_timer' in all cases

Make sure to check that the 'ao_timer' value is valid for both the
high speed and low speed cases in usbduxsigma_ao_cmdtest().

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>