Bluetooth: controller: Fix controller assert at clock rollover

Fix controller assert due to a bug introduced in commit
07270e52 ("Bluetooth: controller: Coding style and
refactoring").

This reverts implementation to original way it was and the
calculation of the ticker expiry will now not overflow the
range of the RTC peripheral, which is a 24 bit counter.

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