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>

ports/linux/dlmstp_linux.c

@@ -384,6 +384,20 @@ uint16_t MSTP_Get_Send(
     return pdu_len;
 }
 
+/**
+ * @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)
+{
+    RS485_Send_Frame(mstp_port, buffer, nbytes);
+}
+
 bool dlmstp_compare_data_expecting_reply(uint8_t *request_pdu,
     uint16_t request_pdu_len,
     uint8_t src_address,