iio: adc: ad9467: check for backend capabilities

iio: adc: ad9467: check for backend capabilities

Add capability checks for operation with backends that do not necessarily
support full set of features, but are otherwise compatible with the device.
This ensures a fully functional device, but with limited capabilities.

Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Tomas Melin <tomas.melin@vaisala.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>