regmap: Add missing map->bus check

The map->bus can be NULL here, add the missing NULL pointer check.

Fixes: d77e745 ("regmap: Add bulk read/write callbacks into regmap_config")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Mark Brown <broonie@kernel.org>
To: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/r/20220509003035.225272-1-marex@denx.de
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: Marek Vasut

drivers/base/regmap/regmap.c

@@ -1839,7 +1839,7 @@ static int _regmap_raw_write_impl(struct regmap *map, unsigned int reg,
 				 map->format.reg_bytes +
 				 map->format.pad_bytes +
 				 val_len);
-	else if (map->bus->gather_write)
+	else if (map->bus && map->bus->gather_write)
 		ret = map->bus->gather_write(map->bus_context, map->work_buf,
 					     map->format.reg_bytes +
 					     map->format.pad_bytes,