uhd: Fix radio_control-related method constness

The const-ness of some radio_control differed between base class and
implementation. This fixes the consistency, but also makes sure these
methods follow the rules for when to make methods 'const'.

The following rules apply:
- Methods that query static capabilities are const. Here, we made
  get_tx_lo_sources() const (the RX version was already const).
- Getters that may have to interact with the device (e.g., peek
  a register) are not const, because the act of peeking is usually also
  non-const. Here, we changed get_rx_lo_export_enabled() to non-const.
- All base classes are fixed such that the derived classes and the base
  classes have the same const-ness. Clang was warning about differences.
  This can cause very tricky bugs, where the radio_control_impl version
  can get called instead of the intended child class.

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