clk: imx: composite-7ulp: Use NULL instead of 0

Address the sparse warnings "
sparse warnings: (new ones prefixed by >>)
>> drivers/clk/imx/clk-composite-7ulp.c:85:24: sparse: sparse:
Using plain integer as NULL pointer
"

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202406220536.JnAncjqz-lkp@intel.com/
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Abel Vesa <abel.vesa@linaro.org>
Link: https://lore.kernel.org/r/20240624024351.488492-1-peng.fan@oss.nxp.com
Signed-off-by: Abel Vesa <abel.vesa@linaro.org>

Author: MrVan

drivers/clk/imx/clk-composite-7ulp.c

@@ -82,7 +82,7 @@ static struct clk_hw *imx_ulp_clk_hw_composite(const char *name,
 	val = readl(reg);
 	if (!(val & PCG_PR_MASK)) {
 		pr_info("PCC PR is 0 for clk:%s, bypass\n", name);
-		return 0;
+		return NULL;
 	}
 
 	if (mux_present) {