net: dsa: b53: Support prepended Broadcom tags

On BCM58xx devices (Northstar Plus), there is an accelerator attached to
port 8 which would only work if we use prepended Broadcom tags. Resolve
that difference in our get_tag_protocol() function by setting the
appropriate tagging protocol in that case. We need to change
b53_brcm_hdr_setup() a little bit now since we can deal with two types
of Broadcom tags.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>