fix(RLC): Count dropped DL SDUs by the RLC

When a downlink SDU was discarded by the RLC Acknowledged Mode entity due to a full transmit buffer (tx_maxsize), the drop statistics were not being updated. The txpdu_dd_pkts and txpdu_dd_bytes counters remained at 0. It should be noted that while the statistics are named txpdu_dd_*, the drop at this stage occurs at the SDU level.