iio: imu: inv_mpu6050: enable i2c aux mux bypass only once

i2c auxiliary mux is done by analog switches. You do not need to
set them for every i2c transfer.
Just set i2c bypass bit at init and do noting in i2c de/select.

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