iio: imu: inv_mpu6050: use set_power_itg function in i2c mux

Set power function is rewritten manually inside i2c mux select.
Better use the already identical existing function.

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