GenericAnalogIn: Check if 'sampling_frequency' is a channel attribute.

[teoperisanu]

An iio device samples using only one samplerate. Sometimes the 'sampling_frequency' attribute is located inside all channels, these sharing it.

TODO:

• [x] check each channel until the attribute is found.
• [ ] add device index/name as parameter

Signed-off-by: Teo Perisanu [email protected]