ixgbe: send MFLCN to ethtool

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: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>