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: b73d9e62 ("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>