Added MSTP extended frame transmit. (#531)

* Added MSTP extended frame transmit to bacnet/datalink/mstp.c and ports/stm32f4xx/dlmstp.c modules.

* Changed RS485_Send_Frame to MSTP_Send_Frame in mstp.c

---------

Co-authored-by: Steve Karg <skarg@users.sourceforge.net>

apps/mstpcap/main.c

@@ -430,6 +430,22 @@ uint16_t MSTP_Get_Send(
     return 0;
 }
 
+/**
+ * @brief Send an MSTP frame
+ * @param mstp_port - port specific data
+ * @param buffer - data to send
+ * @param nbytes - number of bytes of data to send
+ */
+void MSTP_Send_Frame(
+    volatile struct mstp_port_struct_t *mstp_port,
+    uint8_t * buffer,
+    uint16_t nbytes)
+{
+    (void)mstp_port;
+    (void)buffer;
+    (void)nbytes;
+}
+
 uint16_t MSTP_Get_Reply(
     volatile struct mstp_port_struct_t *mstp_port, unsigned timeout)
 { /* milliseconds to wait for a packet */