mavutil: Return mode as 'Mode(msg.custom_mode)' for high-latency streams

Before, when reading high-latency messages that came from a PX4
controller mavutil threw an exception because after the check for PX4,
`msg.base_mode` was read immediately. It was not regarded that the
`HIGH_LATENCY2` message does not contain a `base_mode` field.

This introduces a fix where the mode string for _all_ `HIGH_LATENCY2`
messages is returnes as `Mode(msg.custom_mode)`.

mavutil.py

@@ -2272,14 +2272,19 @@ def mode_mapping_bynumber(mav_type):
 
 def mode_string_v10(msg):
     '''mode string for 1.0 protocol, from heartbeat'''
+    if msg.get_type() == "HIGH_LATENCY2":
+        return f"Mode({msg.custom_mode})"
+
     if msg.autopilot == mavlink.MAV_AUTOPILOT_PX4:
         return interpret_px4_mode(msg.base_mode, msg.custom_mode)
-    if msg.get_type() != 'HIGH_LATENCY2' and not msg.base_mode & mavlink.MAV_MODE_FLAG_CUSTOM_MODE_ENABLED:
+
+    if not msg.base_mode & mavlink.MAV_MODE_FLAG_CUSTOM_MODE_ENABLED:
         return "Mode(0x%08x)" % msg.base_mode
 
     mode_map = mode_mapping_bynumber(msg.type)
     if mode_map and msg.custom_mode in mode_map:
         return mode_map[msg.custom_mode]
+
     return "Mode(%u)" % msg.custom_mode
 
 def mode_string_apm(mode_number):