net: dsa: b53: Enable Broadcom tags for 531x5/539x families

The BCM531x5 and BCM539x families require that the IMP port be enabled
within the management page and that management mode (SM_SW_FWD_MODE) be
turned on. Once this is done, everything works as expected, including
multicast with standalone DSA devices or bridge devices.

Because such switches are frequencly cascaded with other internal
Broadcom switches on which we want to enable Broadcom tags, update
b53_can_enable_brcm_tags() to check the kind of DSA master tagging
protocol being used, if it is one of the two supported Broadcom tagging
protocols, force DSA_TAG_PROTO_NONE.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>