libbladeRF: API change for get/set data transfer timeout

Addresses issue #202.

Fixed name and made return value an operation status rather than
returned value for consistency. We may need to make some
backend-specific calls here in the future, hence this interface
returning status.

The current implementation is not backend-specific, and has been moved
up to the top-level.
libbladeRF: Renamed correction value getter, cleaned up get/set correction