ASoC: stm32: sai: fix master clock naming

Fixes: 8307b2af ("ASoC: stm32: sai: set sai as mclk clock provider")

Fix warning issued by strncat when bound equals to source length.

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