nr rlc: fix in process_control_pdu()

the following:

     if (cur_wait_list == entity->wait_end)
       end_wait_list = prev_wait_list;

was done after cur_wait_list is updated to its new value, but the old value has to be used in this test. So end_wait_list was not set properly.

This was leading to random crashes later on because then entity->wait_list was NULL but entity->wait_end was not (because of the bug fixed by this commit).

The new value for cur_wait_list is also now set using prev_wait_list->next just before the goto, as done in other places in the code.

Also "cur" was removed, for it has no purpose and makes the code harder to understand.