net: bridge: allow SW learn to take over HW fdb entries

Allow to take over an entry which was previously learned via HW when it
shows up from a SW port. This is analogous to how HW takes over SW learned
entries already.

Suggested-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>