iio: adc: Add check for devm_request_threaded_irq

As the potential failure of the devm_request_threaded_irq(),
it should be better to check the return value and return
error if fails.

Fixes: fa659a40

 ("iio: adc: twl6030-gpadc: Use devm_* API family")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Link: https://lore.kernel.org/r/20220224062849.3280966-1-jiasheng@iscas.ac.cn
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>