Compass: change last_update to be in microseconds

libraries/AP_Compass/AP_Compass_HIL.cpp

@@ -16,7 +16,7 @@
 bool AP_Compass_HIL::read()
 {
 	// values set by setHIL function
-	last_update = millis();  // record time of update
+	last_update = micros();  // record time of update
 	return true;
 }
 

libraries/AP_Compass/AP_Compass_HMC5843.cpp

@@ -279,7 +279,7 @@ bool AP_Compass_HMC5843::read()
    mag_y *= calibration[1];
    mag_z *= calibration[2];
 
-   last_update = millis();  // record time of update
+   last_update = micros();  // record time of update
    // rotate and offset the magnetometer values
    // XXX this could well be done in common code...