ASoC: stm32: sai: simplify sync modes management

Use function of_find_device_by_node() to retrieve SAI
synchro provider device and private data.
This allows to remove registration of probed SAI
in a linked list.

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>