regmap: No multi_write support if bus->write does not exist

There is no multi_write support available if we cannot use raw_write.
This is the case if bus->write is not implemented.

This patch adds a condition that we need bus and bus->write so that
can_multi_write is true.
Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/base/regmap/regmap.c

@@ -578,7 +578,7 @@ struct regmap *regmap_init(struct device *dev,
 		map->reg_stride = 1;
 	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;
+	map->can_multi_write = config->can_multi_write && bus && bus->write;
 	map->dev = dev;
 	map->bus = bus;
 	map->bus_context = bus_context;