ASoC: stm32: fix sai driver name initialisation

This patch fixes the sai driver structure overwriting which results in
a cpu dai name equal NULL.

Fixes: 3e086edf

 ("ASoC: stm32: add SAI driver")

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>