iio: adc: ad9467: fix scan type sign

iio: adc: ad9467: fix scan type sign

According to the IIO documentation, the sign in the scan type should be
lower case. The ad9467 driver was incorrectly using upper case.

Fix by changing to lower case.

Fixes: 4606d0f4b05f ("iio: adc: ad9467: add support for AD9434 high-speed ADC")
Fixes: ad6797120238 ("iio: adc: ad9467: add support AD9467 ADC")
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240503-ad9467-fix-scan-type-sign-v1-1-c7a1a066ebb9@baylibre.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>