Fix ABP mode initialization

A modem initialized to factory defaults should be implicitly configured
to operate in the ABP mode with default session keys. This requires
invoking (internal) MLME join to properly initialize the LoRaMac
library. This patch modifies the modem to perform the internal join in
this corner case.

Without this modification, LoRaMac used the wrong application session
key until a mode switch (AT+MODE=1 followed by AT+MODE=0) was performed.

src/lrw.c

@@ -686,9 +686,7 @@ static void set_defaults(void)
 #endif
 
     // The original firmware configures the node in ABP mode by default
-    r.Type = MIB_NETWORK_ACTIVATION;
-    r.Param.NetworkActivation = ACTIVATION_TYPE_ABP;
-    LoRaMacMibSetRequestConfirm(&r);
+    lrw_set_mode(0);
 
     // Disable LoRaWAN certification port by default
     r.Type = MIB_IS_CERT_FPORT_ON;