iio: adc128s052: Fix the error handling path of 'adc128_probe()'

A successful 'regulator_enable()' call should be balanced by a
corresponding 'regulator_disable()' call in the error handling path of the
probe, as already done in the remove function.

Update the error handling path accordingly.

Fixes: 913b8646

 ("iio: adc: Add TI ADC128S052")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/85189f1cfcf6f5f7b42d8730966f2a074b07b5f5.1629542160.git.christophe.jaillet@wanadoo.fr
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>