Bluetooth: controller: Fix llcp_rx check assert

If LL Connection Parameter Request or LL Connection Update
or LL PHY Update procedure is started by the local device
while a LL Length Update Request PDU has been sent by peer
then a Rx node has been stored in the llcp_rx place holder
for generation of Length Update procedure complete.

The failing assert check is incorrect in the above scenario
hence remove. Instead a missing append of the allocated Rx
node to the llcp_rx list has been added to the controller
implementation.

This issue relates to commit d12c53f8 ("Bluetooth:
controller: split: Fix missing data len update event").

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