only use 1M parameters by default

[barbibulle]

Some controllers, like Zephyr-based controllers, don't like it when passing multiple connection parameters in the connection request (even if the PHY is supported). For example, with a Zephyr controller (from the Zephyr HCI USB example), when not specifying explicit parameters in the connection request (which is what most of the tools and examples do), this would lead
to the controller returning HCI_UNSUPPORTED_FEATURE_OR_PARAMETER_VALUE_ERROR.
This change is for only specifying default values for 1M if no explicit preferences are passed (you can still pass arbitrary sets if you want, this only changes the default). It also seems reasonable to pick only 1M by default, since that's the expected baseline for all controllers.