ASoC: AMD: Add NULL pointer check

Fix crash in those platforms whose machine driver does not expose
platform_info. For those platforms we rely on default value and
select I2SSP channel.
Signed-off-by: NAkshu Agrawal <akshu.agrawal@amd.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

sound/soc/amd/acp-pcm-dma.c

@@ -773,6 +773,7 @@ static int acp_dma_hw_params(struct snd_pcm_substream *substream,
 	if (WARN_ON(!rtd))
 		return -EINVAL;
 
+	if (pinfo)
 		rtd->i2s_instance = pinfo->i2s_instance;
 	if (adata->asic_type == CHIP_STONEY) {
 		val = acp_reg_read(adata->acp_mmio,