ixgbe: explicitly disable 100H for x540

100H is not supported on this HW, but the bit is set on the PHY.
This can result in link at 100F when advertising only 1000F.

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>