iio: adc: twl4030: Fix an error handling path in 'twl4030_madc_probe()'

commit 245a396a upstream.

If 'devm_regulator_get()' fails, we should go through the existing error
handling path instead of returning directly, as done is all the other
error handling paths in this function.

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