ASoC: stm32: sai: fix warning in stm32_sai_set_config()

Fix uninitialized warning introduced by
"Move static settings to DAI init" commit
in stm32_sai_set_config() function.

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