staging: comedi: amplc_dio200_common: remove unnecessary 'counter_number' checks

The 'counter_number' in these functions is the comedi channel number from the
chanspec. The comedi core validates the chanspec before calling the driver
functions. Remove the unnecessary checks.

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>