ASoC: Tegra: tegra_pcm_deallocate_dma_buffer: Don't OOPS

ASoC: Tegra: tegra_pcm_deallocate_dma_buffer: Don't OOPS

commit a96edd59b2bc88b3d1ea47e0ba48076d65db9302 upstream.

Not all PCM devices have all sub-streams. Specifically, the SPDIF driver
only supports playback and hence has no capture substream. Check whether
a substream exists before dereferencing it, when de-allocating DMA
buffers in tegra_pcm_deallocate_dma_buffer.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>