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