Fix data race in mux_nr::add_bsr_ce.

srsue/hdr/stack/mac_nr/mux_nr.h

@@ -69,7 +69,8 @@ private:
 
   srsran::mac_sch_pdu_nr tx_pdu; /// single MAC PDU for packing
 
-  enum { no_bsr, sbsr_ce, lbsr_ce } add_bsr_ce = no_bsr; /// BSR procedure requests MUX to add a BSR CE
+  enum bsr_req_t { no_bsr, sbsr_ce, lbsr_ce };
+  std::atomic<bsr_req_t> add_bsr_ce = {bsr_req_t::no_bsr}; /// BSR procedure requests MUX to add a BSR CE
 
   // Mutex for exclusive access
   std::mutex mutex;