Bluetooth: Use hci_conn_params_add within hci_conn_params_set

The hci_conn_params_add function provides the default allocation of
connection parameters. To avoid code duplication, use that code from
hci_conn_params_set to allocate or lookup parameter struct.

As a benefit the connection latency and supervision timeout parameters
are no longer reset to default when calling hci_conn_params_set.

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