bonding: make it possible to have unlimited nested upper vlans

Currently we're limited by a constant level of vlan nestings, and fail to
find anything beyound that level (currently 2).

To fix this - remove the limit of nestings when going through device tree,
and when the end device is found - allocate the needed amount of vlan tags
and return them, instead of found/not found.

CC: Jay Vosburgh <j.vosburgh@gmail.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>