Adding crc_ok[] so that we don't redo LDPC decoding of a segment on
retransmissions if the CRC was correct for the segment in a previous
transmission.

We have to be careful with DTX of round 0, because in this case
nr_ulsch_decoding() will never be called with new_rx == 1, so we
would not reset crc_ok[]. (Hopefully I understand things correctly
here...)