ASoC: rt5677-spi: fix sparse warnings

Fix bugs reported by kbuild test robot

Fixes: a0e0d135 ("ASoC: rt5677: Add a PCM device for streaming hotword via SPI")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Curtis Malainey <cujomalainey@chromium.org>
Link: https://lore.kernel.org/r/20191024184026.183913-1-cujomalainey@chromium.orgSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/rt5677-spi.c

@@ -145,7 +145,7 @@ static int rt5677_spi_hw_free(
 			snd_soc_component_get_drvdata(component);
 
 	mutex_lock(&rt5677_dsp->dma_lock);
-	rt5677_dsp->substream = 0;
+	rt5677_dsp->substream = NULL;
 	mutex_unlock(&rt5677_dsp->dma_lock);
 
 	return snd_pcm_lib_free_vmalloc_buffer(substream);
@@ -362,7 +362,7 @@ static void rt5677_spi_copy_work(struct work_struct *work)
 	mutex_unlock(&rt5677_dsp->dma_lock);
 }
 
-struct page *rt5677_spi_pcm_page(
+static struct page *rt5677_spi_pcm_page(
 		struct snd_soc_component *component,
 		struct snd_pcm_substream *substream,
 		unsigned long offset)