i40e: Use new 40G speeds

The kernel has added SPEED_40000 for ethtool.
Go ahead and use the new #define.

Change-ID: Ic7e16e5c9e91085afe539f11ee1b7668adc4d0ef
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>