iio: adc: ad7380: fix supplies for ad7380-4

ad7380-4 is the only device in the family that does not have an internal
reference. It uses "refin" as a required external reference.
All other devices in the family use "refio"" as an optional external
reference.

Fixes: 737413da ("iio: adc: ad7380: add support for ad738x-4 4 channels variants")
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Link: https://patch.msgid.link/20241022-ad7380-fix-supplies-v3-4-f0cefe1b7fa6@baylibre.com


Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>