ArduSub: create and use a singleton for SRV_Channels

avoid creation of static pointers to objects held within SRV_Channels

ArduSub/ArduSub.cpp

@@ -290,7 +290,7 @@ void Sub::one_hz_loop()
     }
 
     // update assigned functions and enable auxiliary servos
-    SRV_Channels::enable_aux_servos();
+    AP::srv().enable_aux_servos();
 
 #if HAL_LOGGING_ENABLED
     // log terrain data

ArduSub/motors.cpp

@@ -22,7 +22,7 @@ void Sub::motors_output()
         SRV_Channels::calc_pwm();
         SRV_Channels::output_ch_all();
         motors.output();
-        SRV_Channels::push();
+        AP::srv().push();
     }
 }