net: mediatek: add next data pointer coherency protection

The QDMA engine can fail to update the register pointing to the next TX
descriptor if this bit does not get set in the QDMA configuration register.
Not setting this bit can result in invalid values inside the TX rings
registers which will causes TX stalls.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>