ASoC: fsl_rpmsg: initialise pointers to NULL

This fixes the following sparse warnings:

sound/soc/fsl/fsl_rpmsg.c:45:45: sparse: sparse: Using plain integer as NULL pointer
sound/soc/fsl/fsl_rpmsg.c:45:56: sparse: sparse: Using plain integer as NULL pointer

Fixes: b73d9e6 ("ASoC: fsl_rpmsg: Add CPU DAI driver for audio base on rpmsg")
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/1616988868-971-1-git-send-email-shengjiu.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/fsl/fsl_rpmsg.c

@@ -42,7 +42,7 @@ static int fsl_rpmsg_hw_params(struct snd_pcm_substream *substream,
 			       struct snd_soc_dai *dai)
 {
 	struct fsl_rpmsg *rpmsg = snd_soc_dai_get_drvdata(dai);
-	struct clk *p = rpmsg->mclk, *pll = 0, *npll = 0;
+	struct clk *p = rpmsg->mclk, *pll = NULL, *npll = NULL;
 	u64 rate = params_rate(params);
 	int ret = 0;