ixgbe: fix ethtool stats

In latest changes about 64bit stats on 32bit arches,
[commit 28172739f0a276eb8 (net: fix 64 bit counters on 32 bit arches)],
I missed ixgbe uses a bit of magic in its ixgbe_gstrings_stats
definition.

IXGBE_NETDEV_STAT() must now assume offsets relative to
rtnl_link_stats64, not relative do dev->stats.

As a bonus, we also get 64bit stats on ethtool -S

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>