regmap: Re-introduce bulk read support check in regmap_bulk_read()

Support for drivers to define bulk read/write callbacks in regmap_config
was introduced by the commit d77e7456 ("regmap: Add bulk read/write
callbacks into regmap_config"), but this commit wrongly dropped a check
in regmap_bulk_read() to determine whether bulk reads can be done or not.

Before that commit, it was checked if map->bus was set. Now has to check
if a map->read callback has been set.

Fixes: d77e7456

 ("regmap: Add bulk read/write callbacks into regmap_config")
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20220616073435.1988219-2-javierm@redhat.com
Signed-off-by: Mark Brown <broonie@kernel.org>