iio: adc: ad7124: fix possible OOB array access

iio: adc: ad7124: fix possible OOB array access

Reorder the channel bounds check before using it to index into the
channels array in ad7124_release_config_slot(). This prevents reading
past the end of the array.

The value read from invalid memory was not used, so this was mostly
harmless, but we still should not be reading out of bounds in the first
place.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/linux-iio/aPi6V-hcaKReSNWK@stanley.mountain/
Fixes: 9065197e0d41 ("iio: adc: ad7124: change setup reg allocation strategy")
Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>