net: dsa: Move VLAN filtering syncing out of dsa_switch_bridge_leave

net: dsa: Move VLAN filtering syncing out of dsa_switch_bridge_leave

[ Upstream commit 381a730182f1d174e1950cd4e63e885b1c302051 ]

Most of dsa_switch_bridge_leave was, in fact, dealing with the syncing
of VLAN filtering for switches on which that is a global
setting. Separate the two phases to prepare for the cross-chip related
bugfix in the following commit.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>