Bluetooth: controller: Prevent slot reservation update for JIT scheduler

When using the JIT scheduler (CONFIG_BT_CTLR_JIT_SCHEDULING), doing
ticker updates which only change the slot reservation, will fail.

For compatibility, the updates to ull.ticks_slot are preserved, as some
link layer implementations use this to predict collisions.

Signed-off-by: Morten Priess <mtpr@oticon.com>