dmi: Let dmi_walk() users pass private data

At the moment, dmi_walk() lacks flexibility, users can't pass data to
the callback function. Add a pointer for private data to make this
function more flexible.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Matthew Garrett <mjg@redhat.com>
Cc: Roland Dreier <rolandd@cisco.com>