net/i40e: report VLAN filter capability

The rte_eth_dev_set_vlan_offload function will check vlan rx offload
capability, the i40e vf has vlan filter feature but
DEV_RX_OFFLOAD_VLAN_FILTER is not set into the capability, that will
cause setting fail. So need to add this capability in
i40e_vf_representor_dev_infos_get function.

Fixes: e0cb96204b71 ("net/i40e: add support for representor ports")
Cc: stable@dpdk.org
Signed-off-by: Zhihong Peng <zhihongx.peng@intel.com>
Acked-by: Jeff Guo <jia.guo@intel.com>