ASoC: sti: set player private data

Set substream player private data.
substream player private data is used in uni_player_irq_handler to lock,
stop & unlock the stream when interrupt indicates underflow/overflow.
If not set, then segmentation fault occurs.

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