dell-laptop: Only enable rfkill on Latitudes

The rfkill functionality was removed from the dell-laptop driver because it
was causing problems on various non Latitude models, and the blacklist kept
growing and growing. In the thread discussing this Dell mentioned that they
only QA the rfkill acpi interface on Latitudes and indeed there have been
no blacklist entries for Latitudes.

Note that the blacklist contained no Vostros either, and most Vostros have
a hardware switch too, so we could consider supporting Vostros with a
hardware switch too.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>