[SX128x] Use length from startReceive in implicit header mode

src/modules/SX128x/SX128x.cpp

@@ -589,7 +589,11 @@ int16_t SX128x::startReceive() {
 }
 
 int16_t SX128x::startReceive(uint16_t timeout, RadioLibIrqFlags_t irqFlags, RadioLibIrqFlags_t irqMask, size_t len) {
-  (void)len;
+  // in implicit header mode, use the provided length if it is nonzero
+  // otherwise we trust the user has previously set the payload length manually
+  if((this->headerType == RADIOLIB_SX128X_LORA_HEADER_IMPLICIT) && (len != 0)) {
+    this->payloadLen = len;
+  }
   
   // check active modem
   if(getPacketType() == RADIOLIB_SX128X_PACKET_TYPE_RANGING) {