DCM: adjust yaw kp constant down to 0.4

this makes the time constant for compass errors closely match the
timing of the older releases - about 10 seconds for a 1 radian change
in heading

libraries/AP_DCM/AP_DCM.h

@@ -26,7 +26,7 @@ public:
 	// Constructors
 	AP_DCM(IMU *imu, GPS *&gps) :
 		_kp_roll_pitch(0.13),
-		_kp_yaw(0.8),
+		_kp_yaw(0.4),
 		_gps(gps),
 		_imu(imu),
 		_dcm_matrix(1, 0, 0,