Bluetooth: controller: fixing rx node leak on CPR reject of parallel CPR

In case a CPR is intiated but rejected due to CPR active on
other connection, rx nodes are leaked due to retained node not
being properly released.

Signed-off-by: Erik Brockhoff <erbr@oticon.com>
(cherry picked from commit edef1b7c)
Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>