net: dsa: b53: Fix valid setting for MDB entries

When support for the MDB entries was added, the valid bit was correctly
changed to be assigned depending on the remaining port bitmask, that is,
if there were no more ports added to the entry's port bitmask, the entry
now becomes invalid. There was another assignment a few lines below that
would override this which would invalidate entries even when there were
still multiple ports left in the MDB entry.

Fixes: 5d65b64a ("net: dsa: b53: Add support for MDB")
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>