switchdev: fix: erasing too much of vlan obj when handling multiple vlan specs

When adding vlans with multiple IFLA_BRIDGE_VLAN_INFO attrs set in AFSPEC,
we would wipe the vlan obj struct after the first IFLA_BRIDGE_VLAN_INFO.
Fix this by only clearing what's necessary on each IFLA_BRIDGE_VLAN_INFO
iteration.

Fixes: 9e8f4a54

 ("switchdev: push object ID back to object structure")
Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>