AP_OpticalFlow: use millis/micros/panic functions

libraries/AP_OpticalFlow/AP_OpticalFlow_Linux.cpp

@@ -154,7 +154,7 @@ void AP_OpticalFlow_Linux::update(void)
     }
 
     // throttle reads to no more than 10hz
-    uint32_t now = hal.scheduler->millis();
+    uint32_t now = AP_HAL::millis();
     if (now - last_read_ms < 100) {
         return;
     }

libraries/AP_OpticalFlow/OpticalFlow.cpp

@@ -75,7 +75,7 @@ void OpticalFlow::update(void)
         backend->update();
     }
     // only healthy if the data is less than 0.5s old
-    _flags.healthy = (hal.scheduler->millis() - _last_update_ms < 500);
+    _flags.healthy = (AP_HAL::millis() - _last_update_ms < 500);
 }
 
 void OpticalFlow::setHIL(const struct OpticalFlow::OpticalFlow_state &state)

libraries/AP_OpticalFlow/OpticalFlow_backend.cpp

@@ -22,5 +22,5 @@ extern const AP_HAL::HAL& hal;
 void OpticalFlow_backend::_update_frontend(const struct OpticalFlow::OpticalFlow_state &state)
 {
     frontend._state = state;
-    frontend._last_update_ms = hal.scheduler->millis();
+    frontend._last_update_ms = AP_HAL::millis();
 }