Bluetooth: Controller: Fix incorrect elapsed events value

Fix incorrect elapsed events value when LLL event prepare_cb
was invoked but was aborted before anchor point sync. This
caused premature supervision timeouts under applications
configured with CONFIG_BT_CTLR_EVENT_OVERHEAD_RESERVE_MAX=n
and CONFIG_BT_CTLR_PERIPHERAL_RESERVE_MAX=n.

Fixes commit 247037bd ("Bluetooth: Controller: Fix
incorrect elapsed events value").

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