Bluetooth: Store default and maximum LE data length settings

When the controller supports the LE Data Length Extension feature, the
default and maximum data length are read and now stored.

For backwards compatibility all values are initialized to the data
length values from Bluetooth 4.1 and earlier specifications.

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