Set class B flag if pingable

fix #480

Author: sualko

src/lmic/lmic.c

@@ -1889,6 +1889,9 @@ static bit_t buildDataFrame (void) {
     LMIC.frame[OFF_DAT_HDR] = HDR_FTYPE_DAUP | HDR_MAJOR_V1;
     LMIC.frame[OFF_DAT_FCT] = (LMIC.dnConf | LMIC.adrEnabled
                               | (sendAdrAckReq() ? FCT_ADRACKReq : 0)
+#if !defined(DISABLE_PING)
+                              | (LMIC.opmode & OP_PINGABLE) ? FCT_CLASSB : 0
+#endif // ndef DISABLE_PING
                               | (end-OFF_DAT_OPTS));
     os_wlsbf4(LMIC.frame+OFF_DAT_ADDR,  LMIC.devaddr);