AP_Airspeed: use functor macros

Functor is not yet being used but let's make is macro fallback to the
previous Delegate implementation for easy of transition between the two.

libraries/AP_Airspeed/AP_Airspeed_I2C.cpp

@@ -43,7 +43,7 @@ bool AP_Airspeed_I2C::init(void)
     _collect();
     i2c_sem->give();
     if (_last_sample_time_ms != 0) {
-        hal.scheduler->register_timer_process(AP_HAL_MEMBERPROC(&AP_Airspeed_I2C::_timer));
+        hal.scheduler->register_timer_process(FUNCTOR_BIND_MEMBER(&AP_Airspeed_I2C::_timer, void));
         return true;
     }
     return false;