MFLCN register is used to set Rx flow control on parts newer than 82598.

This patch sends the value of MFLCN to ethtool, so it can be used in a
register dump (ethtool -d).
Signed-off-by: NEmil Tantilov <emil.s.tantilov@intel.com>
Tested-by: NStephen Ko <stephen.s.ko@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>