radio,test: fix Coverity warning about dead code

add default in switch instead of using the the END state directly
that is checked before entering the switch. Coverity correctly
reports this as dead code.

lib/src/radio/test/test_radio_rt_gain.cc

@@ -262,8 +262,8 @@ int main(int argc, char** argv)
           continue;
         }
         break;
-      case END:
-        continue;
+      default:
+        break;
     }
 
     // Prepare reception buffers