autotest: remove constant in target_component check when downloading mission

Tools/autotest/common.py

@@ -7574,7 +7574,7 @@ Also, ignores heartbeats not from our target system'''
                 raise NotAchievedException("Received MISSION_ACK while waiting for MISSION_COUNT")
             if m.get_type() != 'MISSION_COUNT':
                 continue
-            if m.target_component != 250: # FIXME: constant?!
+            if m.target_component != self.mav.source_system:
                 continue
             if m.mission_type != mission_type:
                 raise NotAchievedException("Mission count response of incorrect type")