ixgbe: add real-time traffic counters

Just like our other drivers before we can switch ixgbe to
provide real-time packet/byte counters to the stack easily.

Signed-off-by: Ayyappan Veeraiyan <ayyappan.veeraiyan@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>