ixgb: Fix erroneous display of stats by ethtool -S

Commit 5675f221 overlooked the way offsets for netdev stats were considered.
Because of this some of the stats shown by ethtool -S were wrong.
This patch fixes it.

Signed-off-by: Ajit Khaparde <ajitk@serverengines.com>
Signed-off-by: David S. Miller <davem@davemloft.net>