hciattach isn't aware that the card gets reset at system suspend, requiring full reinitialization, so it needs to be restarted. Unfortunately, I haven't found a way to describe such relation using systemd unit relations alone that wouldn't break things, so a system-sleep hook is added to stop the service before going into suspend.
The service itself is also tweaked to better handle errors and to make it restartable (previously it would require the card to be reset before hciattach could work again).