Fix data race in is_pending_tx_end.

srsue/hdr/phy/phy_common.h

@@ -296,7 +296,7 @@ private:
   std::mutex              mtch_mutex;
   std::condition_variable mtch_cvar;
 
-  bool is_pending_tx_end = false;
+  std::atomic<bool> is_pending_tx_end{false};
 
   srsran::radio_interface_phy* radio_h = nullptr;
   srslog::basic_logger&        logger;