Bluetooth: Controller: Fix skipped Periodic Sync radio events

Fix skipped Periodic Advertising Sync radio events due to
increased CPU use during pre-emption in LLL from a Scan
radio event to Periodic Advertising Sync radio event by
increasing EVENT_OVERHEAD_START_US value.

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