net: dsa: Keep the vlan_filtering setting in dsa_switch if it's global

The current behavior is not as obvious as one would assume (which is
that, if the driver set vlan_filtering_is_global = 1, then checking any
dp->vlan_filtering would yield the same result). Only the ports which
are actively enslaved into a bridge would have vlan_filtering set.

This makes it tricky for drivers to check what the global state is.
So fix this and make the struct dsa_switch hold this global setting.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>