net: dsa: b53: Deny enslaving port 7 for 7278 into a bridge

On 7278, port 7 connects to the ASP which should only receive frames
through the use of CFP rules, it is not desirable to have it be part of
a bridge at all since that would make it pick up unwanted traffic that
it may not even be able to filter or sustain.

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