nfp: flower: add missing kdoc

Commit 0115552e ("nfp: remove false positive offloads
in flower vxlan") missed adding kdoc for a new parameter
of nfp_flower_add_offload().

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>