ASoC: fsl_utils: Don't use plain integer as NULL pointer

Fix sparse warning:
sound/soc/fsl/fsl_utils.c:125:31: sparse: warning: Using plain integer as NULL pointer
sound/soc/fsl/fsl_utils.c:125:42: sparse: warning: Using plain integer as NULL pointer

Fixes: 7bad8125

 ("ASoC: fsl_utils: Add function to handle PLL clock source")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1657192806-10569-1-git-send-email-shengjiu.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>