mlxsw: spectrum: Add an helper function to cleanup VLAN entries

VLAN entries on a port can be associated with either a bridge VLAN or a
router port. Before the VLAN entry is destroyed these associations need
to be cleaned up.

Currently, this is always invoked from the function which destroys the
VLAN entry, but next patch is going to skip the destruction of the
default entry when a port in unlinked from a LAG.

The above does not mean that the associations should not be cleaned up,
so add a helper that will be invoked from both call sites.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>