net/bnxt: fix VLAN/VXLAN encapsulation header size

The rte_flow_item_eth and rte_flow_item_vlan items are refined.
The structs do not exactly represent the packet bits captured on the
wire anymore so should only copy real header instead of the whole struct.

Replace the rte_flow_item_* with the existing corresponding rte_*_hdr.

Fixes: 09315fc83861 ("ethdev: add VLAN attributes to ethernet and VLAN items")

Signed-off-by: Xiaoyu Min <jackmin@nvidia.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>