ixgbe: fix 82599 KR downshift coexistence with LESM FW module

Disable KR to KX4/KX downshift on 82599 backplane devices when
LESM (Link Establishment State Machine) is enabled in FW. Those
features cannot co-exist as they both manipulate the same registers.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Acked-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Phillip Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>