firmware: meson_sm: stop using 0 as NULL pointer

Use NULL for NULL pointer to fix the following sparse warning:
drivers/firmware/meson/meson_sm.c:85:24: sparse: warning: Using plain integer as NULL pointer
Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20230110031242.4917-1-miles.chen@mediatek.comSigned-off-by: Neil Armstrong <neil.armstrong@linaro.org>

drivers/firmware/meson/meson_sm.c

@@ -82,7 +82,7 @@ static void __iomem *meson_sm_map_shmem(u32 cmd_shmem, unsigned int size)
 
 	sm_phy_base = __meson_sm_call(cmd_shmem, 0, 0, 0, 0, 0);
 	if (!sm_phy_base)
-		return 0;
+		return NULL;
 
 	return ioremap_cache(sm_phy_base, size);
 }