Bluetooth: controller: split: Fix conditional compile error

Fix conditional compilation error in building the central
only samples as all peripheral structure member accesses in
the controller where not correctly compiled out.

Regression introduced in commit 6d8b1246 ("Bluetooth:
controller: split: Refactor LLL conn structure").

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