ideapad-laptop: Constify DMI table and other r/o variables

Constify the rfkill_blacklist[] DMI table, the ideapad_rfk_data[] table
and the ideapad_attribute_group attribute group. There's no need to have
them writeable during runtime.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: Ike Panhc <ike.pan@canonical.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>