proc_bsr: fix BSR insertion in UL PDUs

the BSR trigger wasn't reset after includinga regular or periodic
BSR inside a PDU. This caused the muxing to include another BSR in the
next UL grant. For very small grants, for example with 6 PRBs, this
causes the UE to send way too many BSRs.

srsue/src/stack/mac/proc_bsr.cc

@@ -255,6 +255,8 @@ bool bsr_proc::generate_bsr(bsr_t* bsr, uint32_t pdu_space)
       timer_periodic.run();
       Debug("BSR:   Started periodicBSR-Timer\n");
     }
+    // reset trigger to avoid another BSR in the next UL grant
+    triggered_bsr_type = NONE;
   }
 
   return send_bsr;