Bluetooth: Controller: Add EVENT_OVERHEAD_START_US when enabling states

Add EVENT_OVERHEAD_START_US to ticks_anchor when enabling
advertising, so the first advertising event is not late due
to calculations for aux packets etc.

Signed-off-by: Troels Nilsson <trnn@demant.com>
Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>