HAL_ChibiOS: end scheduler boost on expected delay

we want timer thread to be higher priority than main thread while in a
long operation

libraries/AP_HAL_ChibiOS/Scheduler.cpp

@@ -498,6 +498,9 @@ void Scheduler::expect_delay_ms(uint32_t ms)
     } else {
         expect_delay_start = AP_HAL::millis();
         expect_delay_length = ms;
+
+        // also put our priority below timer thread if we are boosted
+        boost_end();
     }
 }