Fix build issue and incompatibilites introduced by r283620.

• Re-introduce "device ixgbe" option for kernel configs. This allows users to continue using existing kernel configurations, while still letting if_ix and if_ixv exist to allow ifconfig magic to work. Though the amd64 GENERIC config has "device ix" and "device ixv" instead of "device ixgbe".
  
  This is achieved by making the if_ixgbe module a stub that depends on if_ix and if_ixv.

• Replace if_baudrate assignment with if_initbaudrate() call.

Thanks to jhb@ for the stub module suggestion and pointing out the if_baudrate
error.

Approved by: jfv (mentor)