iio: adc: ad7124 fix indentation issue, remove extra tab

A return statement is indented one level too deeply; clean this
up by removing a tab.
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>