net: dsa: qca8k: use dsa_port's bridge pointer

Now that DSA exposes the bridge device pointer to which a port belongs,
use it when programming the port based VLANs and thus remove the cache.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>