iio: adc: max9611: Fix temperature reading in probe

The max9611 driver reads the die temperature at probe time to validate
the communication channel. Use the actual read value to perform the test
instead of the read function return value, which was mistakenly used so
far.

The temperature reading test was only successful because the 0 return
value is in the range of supported temperatures.

Fixes: 69780a3b ("iio: adc: Add Maxim max9611 ADC driver")
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>