iio: adc: aspeed: Fix error handling path

The labels and branching order of the error path of 'aspeed_adc_probe()'
are broken.
Re-order the labels and goto statements.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>