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.

1 files changed, 2 insertions, 2 deletions

diff --git a/host/lib/rfnoc/radio_control_impl.cpp b/host/lib/rfnoc/radio_control_impl.cpp
index 00cdd12ae..709bbd87d 100644
--- a/host/lib/rfnoc/radio_control_impl.cpp
+++ b/host/lib/rfnoc/radio_control_impl.cpp
@@ -689,7 +689,7 @@ void radio_control_impl::set_rx_lo_export_enabled(bool, const std::string&, cons
         "set_rx_lo_export_enabled is not supported on this radio");
 }
 
-bool radio_control_impl::get_rx_lo_export_enabled(const std::string&, const size_t) const
+bool radio_control_impl::get_rx_lo_export_enabled(const std::string&, const size_t)
 {
     return false;
 }
@@ -709,7 +709,7 @@ std::vector<std::string> radio_control_impl::get_tx_lo_names(const size_t) const
 }
 
 std::vector<std::string> radio_control_impl::get_tx_lo_sources(
-    const std::string&, const size_t)
+    const std::string&, const size_t) const
 {
     return {"internal"};
 }