[link-metrics] add missing initialization of noisefloor for link metr…

…ics (#695)

The commit adds missing initialization of noise floor in link metrics
module. Without this commit, the link margin obtained in link metrics
enhanced-ack probing is incorrect.

src/src/radio.c

@@ -359,6 +359,9 @@ void otPlatRadioSetShortAddress(otInstance *aInstance, uint16_t aShortAddress)
 void nrf5RadioInit(void)
 {
     dataInit();
+#if OPENTHREAD_CONFIG_MLE_LINK_METRICS_SUBJECT_ENABLE
+    otLinkMetricsInit(NRF528XX_RECEIVE_SENSITIVITY);
+#endif
     nrf_802154_init();
 }