Bluetooth: HCI: Add handle to LTK command complete events

The BT_HCI_OP_LE_LTK_REQ_REPLY and BT_HCI_OP_LE_LTK_REQ_NEG_REPLY
command complete events contain a connection handle that serves to
match the command with their corresponding command complete event
when there's more than one pending. For now this just adds the
structure definitions required, and later on code will need to
be added to handle possible mismatches.

Change-Id: I37e88070bc0d4d1b80f085f7e162c30715a82103
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>