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: b4ee8a913e ("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>

sound/soc/fsl/fsl_sai.c

@@ -61,7 +61,7 @@ static inline bool fsl_sai_dir_is_synced(struct fsl_sai *sai, int dir)
 
 static struct pinctrl_state *fsl_sai_get_pins_state(struct fsl_sai *sai, u32 bclk)
 {
-	struct pinctrl_state *state = 0;
+	struct pinctrl_state *state = NULL;
 
 	if (sai->is_pdm_mode) {
 		/* DSD512@44.1kHz, DSD512@48kHz */