iio: imu: st_lsm6dsx: fix PM support for st_lsm6dsx i2c controller

Properly suspend/resume i2c slaves connected to st_lsm6dsx master
controller if the CPU goes in suspended state

Fixes: c91c1c84

 ("imu: st_lsm6dsx: add i2c embedded controller support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>