fm10k: rework vxlan_port offload before adding geneve support

In preparation for adding Geneve Rx offload support, refactor the
current VXLAN offload flow to be a bit more generic so that it will be
easier to add the new Geneve code. The fm10k hardware supports one VXLAN
and one Geneve tunnel, so we will eventually treat the VXLAN and Geneve
tunnels identically. To this end, factor out the code that handles the
current list so that we can use the generic flow for both tunnels in the
next patch.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <Krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>