ASoC: stm32: sai: use devm_of_platform_populate()

Use devm_of_platform_populate() instead of of_platform_depopulate()
to simplify driver code.

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