iio: adc: ad799x: fix probe error handling

Since commit 0f7ddcc1 ("iio:adc:ad799x: Write default config on probe
and reset alert status on probe") the error path is wrong since it
leaves the vref regulator on. Fix this by disabling both regulators.

Fixes: 0f7ddcc1

 ("iio:adc:ad799x: Write default config on probe and reset alert status on probe")
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>