Bluetooth: Enable basics for LE Data Length Extension feature

When the controller supports the new LE Data Length Extension feature
from Bluetooth 4.2 specification, enable the new events and read the
values for default and maxmimum data length supported by the controller.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>