iio:adc:twl6030: Enable measurement of VAC

iio:adc:twl6030: Enable measurement of VAC

VAC needs to be wired up to produce proper measurements,
without this change only near zero values are reported.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Fixes: 1696f36482e7 ("iio: twl6030-gpadc: TWL6030, TWL6032 GPADC driver")
Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Link: https://lore.kernel.org/r/20221217221305.671117-1-andreas@kemnade.info
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>