Bluetooth: controller: Make must-expire runtime configurable

Under CONFIG_BT_TICKER_EXT configuration, the ticker interface has been
extended to support enabling/disabling must-expire scheduling. This
means that conn and slave ticker start calls can now omit must-expire
default-on configuration, relying on LLL updating the mode as needed.

Signed-off-by: Morten Priess <mtpr@oticon.com>