ACM: use ahrs.set_fast_gains() on arm/disarm

ArduCopter/motors.pde

@@ -158,6 +158,9 @@ static void init_arm_motors()
 	// temp hack
 	motor_light = true;
 	digitalWrite(A_LED_PIN, LED_ON);
+
+    // go back to normal AHRS gains
+    ahrs.set_fast_gains(false);
 }
 
 
@@ -181,6 +184,9 @@ static void init_disarm_motors()
 		piezo_beep();
 	}
 	#endif
+
+    // setup fast AHRS gains to get right attitude
+    ahrs.set_fast_gains(true);
 }
 
 /*****************************************

ArduCopter/system.pde

@@ -385,10 +385,14 @@ static void startup_ground(void)
 		#if CLI_ENABLED == ENABLED
 			report_imu();
 		#endif
-		// initialise ahrs (may push imu calibration into the mpu6000 if using that device).
-		ahrs.init();
 	#endif
 
+    // initialise ahrs (may push imu calibration into the mpu6000 if using that device).
+    ahrs.init();
+
+    // setup fast AHRS gains to get right attitude
+    ahrs.set_fast_gains(true);
+
 	// reset the leds
 	// ---------------------------
 	clear_leds();