sky2: don't do GRO on second port

There's something very important I forgot to tell you.
 What?

 Don't cross the GRO streams.
 Why?

 It would be bad.
 I'm fuzzy on the whole good/bad thing. What do you mean, "bad"?

 Try to imagine all the Internet as you know it stopping instantaneously
  and every bit in every packet swapping at the speed of light.
 Total packet reordering.
 Right. That's bad. Okay. All right. Important safety tip. Thanks, Hubert

The simplest way to stop this is just avoid doing GRO on the second port.
Very few Marvell boards support two ports per ring, and GRO is just
an optimization.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>