mlxsw: spectrum: Allow controlling destruction of default port VLAN

The function allows flushing all the existing VLAN entries on a port. It
is invoked when a port is destroyed and when it is unlinked from a LAG.
In the latter case, when moving to the new default VLAN, there will not
be a need to destroy the default VLAN entry.

Therefore, add an argument that allows to control whether the default
port VLAN should be destroyed or not. Currently it is always set to
'true'.

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