ixgbevf: refactor ethtool stats handling

This brings the logic closer to how we handle the stats in ixgbe and it
sets us up for introducing per-queue stats.

Use IXGBEVF_STAT and IXGBEVF_NETDEV_STAT for accessing the driver and
netdev stats respectively. This way we don't have to calculate the
stats based on register values which could lead to the counters not
being initialized properly when the interface is down.

IXGBEVF_QUEUE_STATS_LEN is set to include the number of queues.

Also some defines were renamed to use the IXGBEVF prefix.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>