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

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: 938fd562b974 ("iio: adc: Add support for AD4000")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20241030162013.2100253-2-andriy.shevchenko@linux.intel.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>