iio: ad799x: Fix buffered capture for ad7991/ad7995/ad7999
The data buffer for captured mode for the ad799x driver is allocated in the
update_scan_mode() callback. This callback is not set in the iio_info
struct for the ad7791/ad7995/ad7999, which means that the data buffer is
not allocated when a captured transfer is started. As a result the driver
crashes when the first sample is received. To fix this properly set the
update_scan_mode() callback.