GCS_MAVLink: fixed FTP terminate session error

this caused ftp downloads to intermittently fail. The cause is the FTP
client may ask for a session terminate and then immediately afterwards
a ftp open. The open would fail as the ftp session was considered
active

libraries/GCS_MAVLink/GCS_FTP.cpp

@@ -116,7 +116,11 @@ bool GCS_MAVLINK::send_ftp_reply(const pending_ftp &reply)
         reply.chan,
         0, reply.sysid, reply.compid,
         payload);
+    if (reply.req_opcode == FTP_OP::TerminateSession) {
+        ftp.last_send_ms = 0;
+    } else {
         ftp.last_send_ms = AP_HAL::millis();
+    }
     return true;
 }