mc_att_control: run rate controller first and increase fmu prio by one

The rate controller is now run directly after a gyro publication, and
as soon as it publishes the actuator controls, the output driver (fmu/...)
runs.

Test on a Pixracer:
Reduces fmu control latency from 219us to 134us.
If we run the rate controller last (same order as before, just increase
the prio), the latency is 201us.

CPU load is unchanged.

The drawback is that the attitude to rate setpoint generation is delayed
by one cycle (4ms), but it will be reduced to 1ms as soon as we run at
1kHz.