mixer_multirotor: use reduce_only instead of -1000 for yaw thrust reduction

This is more accurate, because if we use -1000 as lower bound, a very large
yaw command can reduce thrust more than needed.

The difference can be seen in the following example
(with roll/pitch airmode):
p_dot_sp = -1.0 # roll acceleration
q_dot_sp = 0.9 # pitch acceleration
r_dot_sp = -0.9 # yaw acceleration

src/lib/mixer/mixer_multirotor.cpp

@@ -339,7 +339,8 @@ void MultirotorMixer::mix_yaw(float yaw, float *outputs)
 		_tmp_array[i] = _rotors[i].thrust_scale;
 	}
 
-	minimize_saturation(_tmp_array, outputs, _saturation_status, -1000.f);
+	// reduce thrust only
+	minimize_saturation(_tmp_array, outputs, _saturation_status, 0.f, 1.f, true);
 }
 
 unsigned

src/lib/mixer/mixer_multirotor.py

@@ -92,7 +92,10 @@ def mix_yaw(m_sp, u, P, u_min, u_max):
     u_r_dot = P[:,2]
     u_pp = minimize_sat(u_p, u_min, u_max+0.15, u_r_dot)
     u_T = P[:, 3]
-    u_ppp = minimize_sat(u_pp, -1000, u_max, u_T)
+    u_ppp = minimize_sat(u_pp, 0, u_max, u_T)
+    # reduce thrust only
+    if (u_ppp > (u_pp)).any():
+        u_ppp = u_pp
     return u_ppp
 
 def airmode_rp(m_sp, P, u_min, u_max):