iio: imu: inv_mpu6050: switch to use sample rate divider

Instead of storing fifo rate in Hz, store the chip internal sample
rate divider. This will be more useful for timestamping. There
are both equivalent.

Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>