iio: imu: st_lsm6dsx: track hw FIFO buffering with fifo_mask

Track hw FIFO state introducing fifo_mask since now the accel sensor
can be enabled during suspend/resume in order to trigger the wake-up
enabling the FIFO in st_lsm6dsx_resume even if it was disabled before
the suspend.  Hence we must separately track the fifo state.

Fixes: 4c997dfa ("iio: imu: st_lsm6dsx: add wakeup-source option")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Tested-by: Sean Nyekjaer <sean@geanix.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>