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

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>