swcore/mpm: prevent premature frame termination

When high loads of random length traffic was going through the switch, there
were situations that words_total for a new frame was set to
fetch_pg_words(=cur_ll.size) for the previous frame. If fetch_pg_words was a
small number the new frame was reported _done_ soon after it was started in the
OutputBlock. Therefore the rest of the frame was staying in the
async_shrink_fifo making a mess for subsequent frames. The final result was OB
hanging in the frozen_cycle state.