ixgbe: Specific check for 100 Full link speed

This patch specifically checks for 100 Full link speed instead of
assuming we are linked at 100 if not linked at 10G and 1G.

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>