iio: adc: ad9467: add mutex to struct ad9467_state

iio: adc: ad9467: add mutex to struct ad9467_state

When calling ad9467_set_scale(), multiple calls to ad9467_spi_write()
are done which means we need to properly protect the whole operation so
we are sure we will be in a sane state if two concurrent calls occur.

Fixes: ad6797120238 ("iio: adc: ad9467: add support AD9467 ADC")
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20231207-iio-backend-prep-v2-3-a4a33bc4d70e@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>