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.

Fixes: d8dca330 ("staging:iio:ad799x: Preallocate sample buffer")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>