Bluetooth: Host: Fix connection reference leak

If connection reference is acquired from `bt_conn_lookup_addr_le` but
`bt_gatt_ccc_cfg_is_matching_conn` return false the connection was not
unreferenced properly. This commit fix the issue by unreferencing the
connection if the condition is false.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>