Bluetooth: Controller: Fix LLCP event_counter used during current event

Fix LLCP to use event_counter in the current event to
compare instant when trying to check instant passed on
reception of PDU with instant.

Fixes regression in commit e3e410ae ("Bluetooth: Controller:
Fix LLCP event_counter value used during prepare").

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