Bluetooth: controller: Fix CPR procedure regression

This fixes regression introduced in commit a11868fe.

Fixes the following conformance tests:
LL/CON/MAS/BV-32-C [Accepting Connection Parameter Request -
Preferred Periodicity]
LL/CON/MAS/BV-33-C [Accepting Connection Parameter Request -
Preferred Periodicity and preferred anchor points]

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