cxgb: fix stats

patch e0348b9a in mainline.

Fix MAC stats accounting.
Fix get_stats.

Signed-off-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>