AP_OpticalFlow: rename dataflash to logger

libraries/AP_OpticalFlow/OpticalFlow.cpp

@@ -156,12 +156,12 @@ void OpticalFlow::update_state(const OpticalFlow_state &state)
 
 void OpticalFlow::Log_Write_Optflow()
 {
-    AP_Logger *instance = AP_Logger::get_singleton();
-    if (instance == nullptr) {
+    AP_Logger *logger = AP_Logger::get_singleton();
+    if (logger == nullptr) {
         return;
     }
     if (_log_bit != (uint32_t)-1 &&
-        !instance->should_log(_log_bit)) {
+        !logger->should_log(_log_bit)) {
         return;
     }
 
@@ -174,7 +174,7 @@ void OpticalFlow::Log_Write_Optflow()
         body_x          : _state.bodyRate.x,
         body_y          : _state.bodyRate.y
     };
-    instance->WriteBlock(&pkt, sizeof(pkt));
+    logger->WriteBlock(&pkt, sizeof(pkt));
 }