staging: iio: adc: ad7280a: fix overwrite of the returned value

The ret returned from ad7280_chain_setup() is needed to initialize the
slave_num. Don't overwrite it with call to devm_add_action_or_reset().

Fixes: cc9c58ef ("staging: iio: adc: ad7280a: use devm_* APIs")
Signed-off-by: Slawomir Stepien <sst@poczta.fm>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>