Bluetooth: GATT: Fix potential bt_conn reference leak

There was a missing bt_conn_unref() when bt_conn_lookup_addr_le()
returns a connection that's not in BT_CONN_CONNECTED state.

Change-Id: I4c6271d5bf596ea4d9b899e99ce1c7b7c8693f5e
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>