iio: adc: ad7380: fix missing max_conversion_rate_hz on adaq4381-4

Add max_conversion_rate_hz to the chip info for "adaq4381-4". Without
this, the driver fails to probe because it tries to set the initial
sample rate to 0 Hz, which is not valid.

Fixes: bbeaec81 ("iio: ad7380: add support for SPI offload")
Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://patch.msgid.link/20250811-iio-adc-ad7380-fix-missing-max_conversion_rate_hs-on-ad4381-4-v1-1-ffb728d7a71c@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>