dt-bindings: iio: adc: adi,ad7606: fix example

Example uses adi,ad7606-8 as compatible, but adi,sw-mode is not
available for it. So remove this property from example.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>