iio: dac: ad5624r: Fix incorrect handling of an optional regulator.

The naming of the regulator is problematic.  VCC is usually a supply
voltage whereas these devices have a separate VREF pin.

Secondly, the regulator core might have provided a stub regulator if
a real regulator wasn't provided. That would in turn have failed to
provide a voltage when queried. So reality was that there was no way
to use the internal reference.

In order to avoid breaking any dts out in the wild, make sure to fallback
to the original vcc naming if vref is not available.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210627163244.1090296-9-jic23@kernel.org