Bluetooth: Read encryption key size for BR/EDR connections

Since Bluetooth 3.0 there's a HCI command available for reading the
encryption key size of an BR/EDR connection. This information is
essential e.g. for generating an LTK using SMP over BR/EDR, so store
it as part of struct hci_conn.

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