ASoC: bcm: Fix pointer cast warning

The NULL check can be done gracefully without cast.  It fixes a
compile warning like:
  sound/soc/bcm/bcm63xx-pcm-whistler.c:184:6: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

Fixes: 88eb404c ("ASoC: brcm: Add DSL/PON SoC audio driver")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20200330135645.9707-1-tiwai@suse.deSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/bcm/bcm63xx-pcm-whistler.c

@@ -181,7 +181,7 @@ bcm63xx_pcm_pointer(struct snd_soc_component *component,
 	snd_pcm_uframes_t x;
 	struct bcm63xx_runtime_data *prtd = substream->runtime->private_data;
 
-	if ((void *)prtd->dma_addr_next == NULL)
+	if (!prtd->dma_addr_next)
 		prtd->dma_addr_next = substream->runtime->dma_addr;
 
 	x = bytes_to_frames(substream->runtime,