regmap: cache: bail in regmap_async_complete() for bus-less maps

Commit f8bd822c ("regmap: cache: Factor out block sync") made
regcache_rbtree_sync() call regmap_async_complete(), which in turn does
not check for map->bus before dereferencing it.

This causes a NULL pointer dereference on bus-less maps.
Signed-off-by: Daniel Mack <zonque@gmail.com>
Cc: stable@vger.kernel.org [v3.10 only]
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/base/regmap/regmap.c

@@ -1717,7 +1717,7 @@ int regmap_async_complete(struct regmap *map)
 	int ret;
 
 	/* Nothing to do with no async support */
-	if (!map->bus->async_write)
+	if (!map->bus || !map->bus->async_write)
 		return 0;
 
 	trace_regmap_async_complete_start(map->dev);