Bluetooth: Controller: Fix missing first connection event

Fix missing first connection event due to first connection
event ticks_slot of central and peripheral overlapping with
the initiator window ticks_slot and advertising ticks_slot,
respectively.

Use ticker_stop_abs interface so that when the ticker is
stopped the ticks_slot_previous is truncated to the actual
ticks used as requested by the supplied absolute ticks.

Relates to commit 8a294a62 ("Bluetooth: controller:
legacy: Fix missing first conn event").

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