regmap: fix a NULL pointer dereference in __regmap_init

__regmap_init() may receive a NULL `struct regmap_bus *bus' pointer,
for example, from snd_hdac_regmap_init(), and it make sure that it
does not NULL deference `bus`, except around ->max_raw_read and
->max_raw_write initialisation. Add missing check.
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/base/regmap/regmap.c

@@ -579,8 +579,10 @@ struct regmap *regmap_init(struct device *dev,
 	map->use_single_read = config->use_single_rw || !bus || !bus->read;
 	map->use_single_write = config->use_single_rw || !bus || !bus->write;
 	map->can_multi_write = config->can_multi_write && bus && bus->write;
-	map->max_raw_read = bus->max_raw_read;
-	map->max_raw_write = bus->max_raw_write;
+	if (bus) {
+		map->max_raw_read = bus->max_raw_read;
+		map->max_raw_write = bus->max_raw_write;
+	}
 	map->dev = dev;
 	map->bus = bus;
 	map->bus_context = bus_context;