AP_InertialSensor: lower slave rate with fast sampling

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

libraries/AP_InertialSensor/AP_InertialSensor_Invensense.cpp

@@ -814,6 +814,15 @@ void AP_InertialSensor_Invensense::_set_filter_register(void)
             // for logging purposes set the oversamping rate
             _set_accel_oversampling(_accel_instance, MPU_FIFO_DOWNSAMPLE_COUNT/2);
             _set_gyro_oversampling(_gyro_instance, MPU_FIFO_DOWNSAMPLE_COUNT);
+
+            /* set divider for internal sample rate to 0x1F when fast
+             sampling enabled. This reduces the impact of the slave
+             sensor on the sample rate. It ends up with around 75Hz
+             slave rate, and reduces the impact on the gyro and accel
+             sample rate, ending up with around 7760Hz gyro rate and
+             3880Hz accel rate
+             */
+            _register_write(MPUREG_I2C_SLV4_CTRL, 0x1F);
         }
     }