Registering a trigger can fail and the return value of
devm_iio_trigger_register() must be checked. Otherwise undefined behavior
can occur when the trigger is used.

Fixes: 7c0299e8 ("iio: adc: Add support for DLN2 ADC")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20211101133043.6974-1-lars@metafoo.de
Cc: <stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>