iio: imu: st_lsm6dsx: flush hw FIFO before resetting the device

flush hw FIFO before device reset in order to avoid possible races
on interrupt line 1. If the first interrupt line is asserted during
hw reset the device will work in I3C-only mode (if it is supported)

Fixes: 801a6e0a ("iio: imu: st_lsm6dsx: add support to LSM6DSO")
Fixes: 43901008 ("iio: imu: st_lsm6dsx: add support to LSM6DSR")
Reported-by: Mario Tesi <mario.tesi@st.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Vitor Soares <vitor.soares@synopsys.com>
Tested-by: Vitor Soares <vitor.soares@synopsys.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>