iio: adc: ad7124: Add update_scan_mode

iio: adc: ad7124: Add update_scan_mode

The callback .set_channel cannot be used to enable and
disable multiple channels at once as they are presented
in the active_scan_mask.

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/20220322105029.86389-3-alexandru.tachici@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>