tg3: Disable multiple TX rings by default due to hardware flaw

Simple round-robin hardware TX scheduling can cause starvation of TX rings
with small packets when other TX rings have large TSO or jumbo packets.

In the simplest case, consider 2 TCP streams running in opposite
directions.  The TSO TX traffic will hash to one ring and the ACKs for the
incoming data on a different TCP connection will hash to a different TX
ring.  The hardware fetches one complete TSO packet (up to 64K data)
before servicing the other TX ring.  When it gets to the other TX ring, it
will only fetch one packet (64-byte ACK packet in this case).  After that,
it will switch back to the 1st ring filled with more TSO packets.  Because
only one ACK can go out roughly every 500 usec in this case, the incoming
data rate becomes very low.

Update version to 3.125.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>