regmap: Fix regmap_can_raw_write check

This function is missing a check if map->bus->write is implemented. If
it is not implemented arbitrary raw writes are not possible.
Signed-off-by: NMarkus Pargmann <mpa@pengutronix.de>
Signed-off-by: NMark Brown <broonie@kernel.org>

drivers/base/regmap/regmap.c

@@ -1382,7 +1382,8 @@ int _regmap_raw_write(struct regmap *map, unsigned int reg,
  */
 bool regmap_can_raw_write(struct regmap *map)
 {
-	return map->bus && map->format.format_val && map->format.format_reg;
+	return map->bus && map->bus->write && map->format.format_val &&
+		map->format.format_reg;
 }
 EXPORT_SYMBOL_GPL(regmap_can_raw_write);