iio: imu: inv_mpu6050: clean set_power_itg and fix usage

Rewrite set_power_itg. Failing when turning power off is
no more decreasing the counter now and sleeping only happens
when effectively turning the chip on. Fix also usage in init
function (setting power on one time is sufficient to ensure
chip is effectively on).

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