Bluetooth: controller: Workaround CPR procedure collision at CPU instant

This is a workaround for IOP issue, where peer rejects LLCP Slave
Connection Parameter Request with LMP Error Transaction Collision
error code even if previous request is complete at the instant.

Relates to #15366.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>
Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>