Can't ping:TSE+DP83848K+RMII

The mac_rx_dv is the "data valid" signals and indicates when the data on mac_rxd[] is valid. It should be connected to the mac's m_rx_en port, not m_rx_col. But this doesn't explain why you don't see anything on SignalTap. Put a positive edge trigger on mac_rx_dv, and if it never triggers, it means either that the PHY isn't receiving a packet or that it isn't connected correctly to the FPGA. In that case you can try to put some Signaltap probes on the rmii side of the interface and see if anything is happening there.