ixgbe: fix the display of DCB control stats in ethtool

Priority flow contol statistics for Data Center Bridging (DCB) weren't
included in ethtool.  This patch adds them.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Eric W Multanen <eric.w.multanen@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>