Adding 8 bytes (sky2->netdev->mtu + ETH_HLEN + VLAN_HLEN + 8
(EDSA_HLEN)) does not fix the error, because the interface keep
having a maximum length of 1518 bytes (sky2->netdev->mtu + ETH_HLEN
+ VLAN_HLEN).

Did you check the value being written to here:

        /*
         * The receiver hangs if it receives frames larger than the
         * packet buffer. As a workaround, truncate oversize frames, but
         * the register is limited to 9 bits, so if you do frames > 2052
         * you better get the MTU right!
         */
        thresh = sky2_get_rx_threshold(sky2);
        if (thresh > 0x1ff)
                sky2_write32(hw, SK_REG(sky2->port, RX_GMF_CTRL_T), RX_TRUNC_OFF);
        else {
                sky2_write16(hw, SK_REG(sky2->port, RX_GMF_TR_THR), thresh);
                sky2_write32(hw, SK_REG(sky2->port, RX_GMF_CTRL_T), RX_TRUNC_ON);
        }


What is thresh?

     Andrew