rrc_meas: disable layer3 filtering until quantity config is received

the filter alpha was initialized to zero by default which causes an
issue because the first measurement for a cell can't be updated,
because the filtering function will always return the current value.

According to 36.331 Sec 5.5.3.2 Note 2, a k-value of 0 should turn
off filtering, which should be used as the default value until
an update is received from the network.

srsue/hdr/stack/rrc/rrc_meas.h

@@ -138,7 +138,7 @@ private:
     std::map<uint32_t, meas_obj_eutra_s>     measObjectsList;  // Uses MeasObjectId as key
     std::map<uint32_t, report_cfg_eutra_s>   reportConfigList; // Uses ReportConfigId as key
 
-    phy_quant_t filter_a        = {};
+    phy_quant_t filter_a = {1.0, 1.0}; // disable filtering until quantityConfig is received (see Sec. 5.5.3.2 Note 2)
     float       s_measure_value = 0.0;
 
     // trigger counters. First key is measId, second key is cell id (pci)