ASoC: spear: spdif_out: Staticize unexported function

The spdif_soc_dai_probe function is only used in spdif_out.c, so make it static.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Rajeev Kumar <rajeev-dlh.kumar@st.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>