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, 1 insertions, 2 deletions

diff --git a/host/lib/usrp/x300/x300_radio_control.cpp b/host/lib/usrp/x300/x300_radio_control.cpp
index 3ab60e661..c6750e397 100644
--- a/host/lib/usrp/x300/x300_radio_control.cpp
+++ b/host/lib/usrp/x300/x300_radio_control.cpp
@@ -696,8 +696,7 @@ public:
         }
     }
 
-    bool get_rx_lo_export_enabled(
-        const std::string& name, const size_t chan) const override
+    bool get_rx_lo_export_enabled(const std::string& name, const size_t chan) override
     {
         fs_path rx_fe_fe_root = get_db_path("rx", chan);