ixgbevf: Add support to recognize 100mb link speed

The X540 10Gig controller is capable of linking at 100Mbits - add
support for reporting that link speed.

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>