dt-bindings: iio: adc: ad7192: Fix 'single-channel' constraints

dt-bindings: iio: adc: ad7192: Fix 'single-channel' constraints

The 'single-channel' property is an uint32, not an array, so 'items' is
an incorrect constraint. This didn't matter until dtschema recently
changed how properties are decoded. This results in this warning:

Documentation/devicetree/bindings/iio/adc/adi,ad7192.example.dtb: adc@0: \
  channel@1:single-channel: 1 is not of type 'array'

Fixes: caf7b7632b8d ("dt-bindings: iio: adc: ad7192: Add AD7194 support")
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240723230904.1299744-1-robh@kernel.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>