regmap: cache: Use correct type of the rb_for_each() parameter

Compiler is not happy:

  regcache.c:410:9: warning: Using plain integer as NULL pointer

Replace integer 0 by NULL.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240606164717.3031107-3-andriy.shevchenko@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/base/regmap/regcache.c

@@ -407,7 +407,7 @@ int regcache_sync(struct regmap *map)
 	 * have gone out of sync, force writes of all the paging
 	 * registers.
 	 */
-	rb_for_each(node, 0, &map->range_tree, rbtree_all) {
+	rb_for_each(node, NULL, &map->range_tree, rbtree_all) {
 		struct regmap_range_node *this =
 			rb_entry(node, struct regmap_range_node, node);