iio: adc: ad7780: remove IIO_CHAN_INFO_SAMP_FREQ support

The `ad7780` driver does not implement setting/getting the sampling
frequency.
For the ad7780/ad7781 devices, the control is done via an external pin,
and the ad7170/ad7171 devices have a fixed sampling rate (so, no control).

For these devices, and similar other that may be added later on,
a AD_SD_CHANNEL_NO_SAMPLE_FREQ() macro has been added, which doesn't set
the IIO_CHAN_INFO_SAMP_FREQ flag.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>