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: 4606d0f4 ("iio: adc: ad9467: add support for AD9434 high-speed ADC")
Fixes: ad679712 ("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>