altera tse: Error-Bit on tx-avalon-stream always set.

The Error-Bit on the avalon streaming interface of the
tx-dma-channel was always set. In SGMII configurations
this leads to error-symbols on the PCS and packet-rejection
on the receiver side (e.g. SGMII/1000Base-X connected switch).

This only applies to the tse-configuration with MSGDMA.

This issue was detected and fixed on a custom board with
a direct connection to a Marvell switch in SGMII-PHY-Mode.
(incl. custom patches for SGMII-PCS).

According to the datasheet if ff_tx_err (avalon-streaming)
is set it is forwarded to gm_tx_err. As a result the PCS
is forwarding the error by sending a "/V/"-caracter.

Signed-off-by: Andreas Oetken <ennoerlangen@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>