Bluetooth: host: Fix CCC cfg not cleared when overwriting oldest bond

Fix CCC cfg not cleared when overwriting oldest bond. Calling
bt_unpair with a pointer to the key will result in the key addr being
memset to zero and bt_gatt_clear is called with an zero-set address.
This happens because unpair (hci_core.c) calls bt_keys_clear before
calling bt_gatt_clear.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>