ixgbe: add ethtool counters for OS2BMC

OS2BMC registers are available for X540.
This patch adds ethtool counters based on those registers.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Evan Swanson <evan.swanson@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>