Sub: fix format-string compilation warning

../../ArduSub/commands_logic.cpp: In member function 'void Sub::do_nav_delay(const AP_Mission::Mission_Command&)':
../../ArduSub/commands_logic.cpp:485:84: warning: format '%u' expects argument of type 'unsigned int', but argument 4 has type 'long unsigned int' [-Wformat=]
     gcs().send_text(MAV_SEVERITY_INFO, "Delaying %u sec",nav_delay_time_max_ms/1000);
                                                                                    ^

ArduSub/commands_logic.cpp

@@ -482,7 +482,7 @@ void Sub::do_nav_delay(const AP_Mission::Mission_Command& cmd)
         // absolute delay to utc time
         nav_delay_time_max_ms = AP::rtc().get_time_utc(cmd.content.nav_delay.hour_utc, cmd.content.nav_delay.min_utc, cmd.content.nav_delay.sec_utc, 0);
     }
-    gcs().send_text(MAV_SEVERITY_INFO, "Delaying %u sec",nav_delay_time_max_ms/1000);
+    gcs().send_text(MAV_SEVERITY_INFO, "Delaying %u sec", (unsigned)(nav_delay_time_max_ms/1000));
 }
 
 #if NAV_GUIDED == ENABLED