iio: imu: st_lsm6dsx: modify st_lsm6dsx_flush_fifo and st_lsm6dsx_set_fifo_mode scope

Remove static qualifier from st_lsm6dsx_flush_fifo() and
st_lsm6dsx_set_fifo_mode() in order to use them in system sleep pm support

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>