net/nbl: add dummy VLAN offload configuration

net/nbl: add dummy VLAN offload configuration

Implement the vlan_offload_set device operation to allow
enabling or disabling VLAN offloads through the API.

rte_eth_dev_set_vlan_offload() calls the driver to update the hardware
configuration. Because this driver performs VLAN stripping entirely in
software, only a dummy implementation is needed to avoid returning an
error.

Validation of unsupported or invalid flags is already handled in ethdev.

Fixes: 9d7757dce874 ("net/nbl: simulate VLAN offload")
Signed-off-by: Dimon Zhao <dimon.zhao@nebula-matrix.com>