When snd_pcm_stop() is called in interrupt routine,
substream context may have already been released.
Add protection on substream context.

Fixes: 03e4d5d5 ("ASoC: stm32: Add SPDIFRX support")
Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Link: https://lore.kernel.org/r/20191204154333.7152-3-olivier.moysan@st.comSigned-off-by: Mark Brown <broonie@kernel.org>