ASoC: stm32: spdifrx: fix race condition in irq handler

ASoC: stm32: spdifrx: fix race condition in irq handler

commit 86e1956af4c863d653136fd6e5694adf2054dbaa upstream.

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

Fixes: 03e4d5d56fa5 ("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.com
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>