iio: imu: inv_mpu6050: use i2c mux only for chip with i2c aux bus

icm20608 does not have i2c aux bus, so it does not make sense to
allocate an i2c mux for this chip.

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