ASoC: stm32: sai: add missing put_device()

The of_find_device_by_node() takes a reference to the underlying device
structure, we should release that reference.

Fixes: 7dd0d835 ("ASoC: stm32: sai: simplify sync modes management")
Signed-off-by: Wen Yang <yellowriver2010@hotmail.com>
Acked-by: Olivier Moysan <olivier.moysan@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>