Fix build issue and incompatibilites introduced by r283620.

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)

git-svn-id: svn://svn.freebsd.org/base/stable/10@283668 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f