dt-bindings: iio: adc: adi,ad7292: Fix additionalProperties on channel nodes

"additionalProperties: true" is only for incomplete schemas such as bus
child nodes in a bus's schema. That doesn't apply to the "channel" nodes
in the adi,ad7292 binding, so fix additionalProperties to be false.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20230926164357.100325-1-robh@kernel.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>