STM32WL fix set preamble length to 8

Author: hallard

connectivity/lorawan/mbed_lib.json

@@ -102,7 +102,8 @@
     "target_overrides": {
         "STM32WL": {
             "wakeup-time": 3,
-            "max-sys-rx-error": 10
+            "max-sys-rx-error": 10,
+            "downlink-preamble-length": 8
         }
     }
 }