cxgb4: fix for new ndo_get_stats64 signature

The change to ndo_get_stats64 in "net: fix 64 bit counters on 32 bit arches"
missed cxgb4.  Fix it.

Signed-off-by: Dimitris Michailidis <dm@chelsio.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>