i40e: display correct UDP tunnel type name

The i40e driver attempts to display the UDP tunnel name by doing a check
against the type, where for non-zero types we use "vxlan" and for zero
type we use "geneve". This is not future proof, because if new tunnel
types get added, we'll incorrectly label them. It also depends on the
value of UDP_TUNNEL_TYPE_GENEVE == 0, which is brittle.

Instead, replace this with a function that can return a constant string
depending on the type. For now we'll use "unknown" for types we don't
know about, and we can expand this in the future if new types get added.

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>