iio: imu: inv_mpu6050: add comment about frequencies

Although the driver allows frequencies between 4 and 1000 Hz, only the
frequencies advertised in the available frequencies file are backed
properly by a low-pass filter to prevent aliasing, so it's best to use
them. Since this is not obvious to the user, add a comment explaining
what's going on.

Signed-off-by: Martin Kelly <mkelly@xevo.com>
Acked-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>