regcache: flat: Require max_registers to be set

If max_register is unset, regcache_flat_get_index will return 0 and only
memory for 1 unsigned int will be allocated, resulting in writing out
of bounds.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/base/regmap/regcache-flat.c

@@ -27,7 +27,7 @@ static int regcache_flat_init(struct regmap *map)
 	int i;
 	unsigned int *cache;
 
-	if (!map || map->reg_stride_order < 0)
+	if (!map || map->reg_stride_order < 0 || !map->max_register)
 		return -EINVAL;
 
 	map->cache = kcalloc(regcache_flat_get_index(map, map->max_register)