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

Fix sparse warning:
sound/soc/fsl/fsl_sai.c:64:39: sparse: warning: Using plain integer as NULL pointer

Fixes: b4ee8a91 ("ASoc: fsl_sai: Add pinctrl operation for PDM and DSD")
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1658399393-28777-2-git-send-email-shengjiu.wang@nxp.com


Signed-off-by: Mark Brown <broonie@kernel.org>