Bluetooth: Controller: Fix Periodic Adv EVENT_OVERHEAD_START_US jitter

As EVENT_OVERHEAD_START_US offset is used in ticks unit in
LLL, ULL scheduling using ticker should also use ticks unit
for EVENT_OVERHEAD_START_US when reducing the first Periodic
Advertising event preparation.

Relates to commit 858dc7fa ("Bluetooth: controller: Fix
EVENT_OVERHEAD_START_US jitter").

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