fix TX PCS 16-bit to always catch proper tx timestamp
When oob.valid, ep_tx_pcs_16bit should wait for the U_TX_FIFO to
empty in order to catch the proper timestamp for the oob signaled
packet.
8-bit version of the PCS was already working this way so we've never
seen the problem before porting WRPC to Kintex-7.
Done with commit 0e2172f.