Bluetooth: controller: Fix CPR procedure's Conn Upd initiation

Fix Connection Parameter Request Procedure's Connection
Update Procedure initiation to calculate the offset rather
than selecting offsets from an out-of-bound memory area.

The symptoms of the bug was noticed as a supervision timeout
due to use of incorrect offset communicated to peer and a
wrong offset used in scheduling the connection events.

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