iio: adc: twl4030: Disable the vusb3v1 rugulator in the error handling path of 'twl4030_madc_probe()'

Commit 7cc97d77 has introduced a call to 'regulator_disable()' in the
.remove function.
So we should also have such a call in the .probe function in case of
error after a successful 'regulator_enable()' call.

Add a new label for that and use it.

Fixes: 7cc97d77 ("iio: adc: twl4030: Fix ADC[3:6] readings")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>