iio: adc: palmas: fix off by one bugs

Valid values for "adc_chan" are zero to (PALMAS_ADC_CH_MAX - 1).
Smatch detects some buffer overflows caused by this:
drivers/iio/adc/palmas_gpadc.c:721 palmas_gpadc_read_event_value() error: buffer overflow 'adc->thresholds' 16 <= 16
drivers/iio/adc/palmas_gpadc.c:758 palmas_gpadc_write_event_value() error: buffer overflow 'adc->thresholds' 16 <= 16

The effect of this bug in other functions is more complicated but
obviously we should fix all of them.

Fixes: a99544c ("iio: adc: palmas: add support for iio threshold events")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/14fee94a-7db7-4371-b7d6-e94d86b9561e@kili.mountain
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: Dan Carpenter

drivers/iio/adc/palmas_gpadc.c

@@ -547,7 +547,7 @@ static int palmas_gpadc_read_raw(struct iio_dev *indio_dev,
 	int adc_chan = chan->channel;
 	int ret = 0;
 
-	if (adc_chan > PALMAS_ADC_CH_MAX)
+	if (adc_chan >= PALMAS_ADC_CH_MAX)
 		return -EINVAL;
 
 	mutex_lock(&adc->lock);
@@ -595,7 +595,7 @@ static int palmas_gpadc_read_event_config(struct iio_dev *indio_dev,
 	int adc_chan = chan->channel;
 	int ret = 0;
 
-	if (adc_chan > PALMAS_ADC_CH_MAX || type != IIO_EV_TYPE_THRESH)
+	if (adc_chan >= PALMAS_ADC_CH_MAX || type != IIO_EV_TYPE_THRESH)
 		return -EINVAL;
 
 	mutex_lock(&adc->lock);
@@ -684,7 +684,7 @@ static int palmas_gpadc_write_event_config(struct iio_dev *indio_dev,
 	int adc_chan = chan->channel;
 	int ret;
 
-	if (adc_chan > PALMAS_ADC_CH_MAX || type != IIO_EV_TYPE_THRESH)
+	if (adc_chan >= PALMAS_ADC_CH_MAX || type != IIO_EV_TYPE_THRESH)
 		return -EINVAL;
 
 	mutex_lock(&adc->lock);
@@ -710,7 +710,7 @@ static int palmas_gpadc_read_event_value(struct iio_dev *indio_dev,
 	int adc_chan = chan->channel;
 	int ret;
 
-	if (adc_chan > PALMAS_ADC_CH_MAX || type != IIO_EV_TYPE_THRESH)
+	if (adc_chan >= PALMAS_ADC_CH_MAX || type != IIO_EV_TYPE_THRESH)
 		return -EINVAL;
 
 	mutex_lock(&adc->lock);
@@ -744,7 +744,7 @@ static int palmas_gpadc_write_event_value(struct iio_dev *indio_dev,
 	int old;
 	int ret;
 
-	if (adc_chan > PALMAS_ADC_CH_MAX || type != IIO_EV_TYPE_THRESH)
+	if (adc_chan >= PALMAS_ADC_CH_MAX || type != IIO_EV_TYPE_THRESH)
 		return -EINVAL;
 
 	mutex_lock(&adc->lock);