Bluetooth: controller: Improved continuous scanning

Refactored the high frequency clock preparation advanced
feature to improve radio utilization during continuous
scanning.

The inter-event timespace value now considers the reserved
timespace while determining if the high frequency clock
will be retained. This reduces the preparation time, hence
increased radio use inside scan window.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>