regmap: Allow rbtree to cache zero default values

Ensure that when we start up in cache only mode we can store defaults of
zero, otherwise if the hardware is unavailable we won't be able to read.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>

drivers/base/regmap/regcache-rbtree.c

@@ -252,9 +252,6 @@ static int regcache_rbtree_write(struct regmap *map, unsigned int reg,
 		regcache_rbtree_set_register(rbnode, reg_tmp, value,
 					     map->cache_word_size);
 	} else {
-		/* bail out early, no need to create the rbnode yet */
-		if (!value)
-			return 0;
 		/* look for an adjacent register to the one we are about to add */
 		for (node = rb_first(&rbtree_ctx->root); node;
 		     node = rb_next(node)) {