modules/swcore: one more bugfix preventing the transfer FSM to be stuck forever in S_DROP on high traffic load

This time only one port was getting stuck (transfer FSM stuck in S_DROP)
once in a while during the 18-port 100% load snake test. In situations
when setting the usecnt was taking some longer time, it may happen that
RCV_FSM was requesting force_free and force_free was done before TP_FSM
went to S_DROP. In that case I register the mmu_force_free_done_i signal
in S_SET_USECNT so that it can be used later in S_DROP (if needed).