iio: adc: stm32: fix bad error check on max_channels

Fix a bad error check when counting 'st,adc-channels' array elements.
This is seen when all channels are in use simultaneously.

Fixes: 64ad7f64 ("iio: adc: stm32: introduce compatible data cfg")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>