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

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

BugLink: http://bugs.launchpad.net/bugs/1723145
commit 4fb840c95f82652cece7352be9080884cafb92a0 upstream.

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

Fixes: 64ad7f643 ("iio: adc: stm32: introduce compatible data cfg")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>