iio: adc: ad7380: fix adi,gain-milli property parsing

iio: adc: ad7380: fix adi,gain-milli property parsing

Change the data type of the "adi,gain-milli" property from u32 to u16.
The devicetree binding specifies it as uint16, so we need to read it as
such to avoid an -EOVERFLOW error when parsing the property.

Fixes: c904e6dcf402 ("iio: adc: ad7380: add support for adaq4370-4 and adaq4380-4")
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250619-iio-adc-ad7380-fix-adi-gain-milli-parsing-v1-1-4c27fb426860@baylibre.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>