regcache: Make sure we sync register 0 in an rbtree cache

Most of the current users have register 0 as a volatile register or don't
have a register 0 so it's not been apparent that it's not getting synced.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

drivers/base/regmap/regcache-rbtree.c

@@ -377,7 +377,7 @@ static int regcache_rbtree_sync(struct regmap *map)
 
 			/* Is this the hardware default?  If so skip. */
 			ret = regcache_lookup_reg(map, i);
-			if (ret > 0 && val == map->reg_defaults[ret].def)
+			if (ret >= 0 && val == map->reg_defaults[ret].def)
 				continue;
 
 			map->cache_bypass = 1;