iio: adc: qcom-spmi-adc5: Verify channel numbers from DT

The driver only defines a subset of all possible ADC channels. Channel
numbers read from the device tree are accepted as long as they don't
exceed a max value, even when no channel definition exists. Add a
check to abort initialization in this case.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>