Bluetooth: Provide function to create and set connection parameters

In some cases it is useful to not overwrite connection parametes and
instead just create default ones if they don't exist. This function
does exactly that. hci_conn_params_add will allow to create new
default connection parameters. hci_conn_params_set will set the
values and also create new parameters if they don't exist.

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