regmap: Add missing map->bus check

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

Fixes: d77e7456

 ("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>