iio: adc: ad4000: Check for error code from devm_mutex_init() call

Even if it's not critical, the avoidance of checking the error code
from devm_mutex_init() call today diminishes the point of using devm
variant of it. Tomorrow it may even leak something. Add the missed
check.

Fixes: 938fd56 ("iio: adc: Add support for AD4000")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

Author: andy-shev

drivers/iio/adc/ad4000.c

@@ -637,7 +637,9 @@ static int ad4000_probe(struct spi_device *spi)
 	indio_dev->name = chip->dev_name;
 	indio_dev->num_channels = 1;
 
-	devm_mutex_init(dev, &st->lock);
+	ret = devm_mutex_init(dev, &st->lock);
+	if (ret)
+		return ret;
 
 	st->gain_milli = 1000;
 	if (chip->has_hardware_gain) {