iio: adc: ad7124: Don't create more channels than the driver can handle

iio: adc: ad7124: Don't create more channels than the driver can handle

The ad7124-4 and ad7124-8 both support 16 channel registers and assigns
each channel defined in dt statically such a register. While the driver
could be a bit more clever about this, it currently isn't and specifying
more than 16 channels yields broken behaviour. So just refuse to bind in
this situation.

Fixes: b3af341bbd96 ("iio: adc: Add ad7124 support")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://patch.msgid.link/1b9a5a1d334e5501294f7f9f9d5893f1cdf1b0ec.1733504533.git.u.kleine-koenig@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>