Make libnet_get_hwaddr() work with 802.1q interfaces in bpf (BSD).

This fixes a regression in introduced in
27282bb5a1ab78210245e226c4c82605b00a12bb.
Merge pull request #32 from ThomasHabets/get-vlan-mac

Make libnet_get_hwaddr() work with 802.1q interfaces in bpf (BSD).