Bluetooth: Controller: Fix BT_CTLR_SCAN_AUX_SYNC_RESERVE_MIN

Fix BT_CTLR_SCAN_AUX_SYNC_RESERVE_MIN such that event is not
aborted when near supervision timeout conditions.

Relates to commit ddf04997 ("Bluetooth: Controller: Add
abort fairness in overlapping Periodic Sync").

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