iio: adc: ad7192: Add update_scan_mode

The callback .set_channel cannot be used to enable multiple
channels at once, only one is allowed simultaneously.

By adding an update_scan_mode callback, every time the
continuous mode is activated, channels will be enabled/disabled
accordingly.

Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Link: https://lore.kernel.org/r/20220322105029.86389-4-alexandru.tachici@analog.com


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