mc_att_control_main: zero minimal thrust when landed

but this time compared to
2fbb70d9ca
the minimum thrust is ramping to not produce a thrust jump.

The better long term solution will be to always have airmode but
with the effect limited to a magnitude of MPC_MANTHR_MIN.

src/lib/slew_rate/SlewRate.hpp

@@ -49,6 +49,7 @@ class SlewRate
 {
 public:
 	SlewRate() = default;
+	SlewRate(Type initial_value) { setForcedValue(initial_value); }
 	~SlewRate() = default;
 
 	/**