net: vlan: Fix net_eth_get_vlan_tag() to check correct interface

The network interface parameter for net_eth_get_vlan_tag() should
be the VLAN interface so use the search loop properly.
Earlier the main interface could be checked.

Add also test cases for this so that we can catch that the func
works properly.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>