On an invensense IMU with fast sampling we need to lower the slave
sample rate for slave sensors such as the built-in AK8963 compass on a
MPU9250.
The slave rate is set as a multiple of the main rate, so it needs to be
much lower for fast sampling. If we leave it high then it greatly
impacts on IMU sample rate.
Without this change a MPU9250 with fast sampling and a compass enabled
will give a gyro rate of around 7200Hz. With this change it gets 7760Hz
Andrew Tridgell
bfd8d151c8