Bluetooth: controller: split: Fix shell app regression

Fix shell app regression due to stale tmp role implementation
not fully being removed in commit 4c77bf61 ("Bluetooth:
controller: Remove redundant tmp role implementation").

The regression caused ticker indexes outside of allocations to
be accessed causing application faults and memory corruptions.

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