i40e: explicitly fail on extended MAC field for ethtool_rx_flow_spec

Although we will fail the filter later due to checking flow_type which
will have a bogus invalid type, it is possible future refactoring will
remove this hidden failure case. Avoid a possible issue in the future by
explicitly checking the flow type at the start.

Change-Id: Ia98eb26f7b93ccbe38c7141e8f203ef496fc6598
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>