samples: net: vlan: Add IPv6 prefix config to each vlan-iface

Without the IPv6-prefix set, the net_if_ipv6_addr_onlink() check
will always fail. The packets with DIP in the local link will go
to 'try_send' code branch, which is not expected.

Signed-off-by: Shrek Wang <shrek.wang@nxp.com>