iio: imu: inv_mpu6050: update LPF bandwidth settings

As every chip has some little variant in LPF bandwidth values,
use common values that are working for all chips.
Simplify the LPF setting function.

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