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: a99544c6 ("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>