net: bridge: vlan: check for errors from __vlan_del in __vlan_flush

If the following call path returns an error from switchdev:

nbp_vlan_flush
-> __vlan_del
   -> __vlan_vid_del
      -> br_switchdev_port_vlan_del
-> __vlan_group_free
   -> WARN_ON(!list_empty(&vg->vlan_list));

then the deletion of the net_bridge_vlan is silently halted, which will
trigger the WARN_ON from __vlan_group_free().

The WARN_ON is rather unhelpful, because nothing about the source of the
error is printed. Add a print to catch errors from __vlan_del.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>