net: dsa: qca8k: fix mdb add/del case with 0 VID

The qca8k switch doesn't support using 0 as VID and require a default
VID to be always set. MDB add/del function doesn't currently handle
this and are currently setting the default VID.

Fix this by correctly handling this corner case and internally use the
default VID for VID 0 case.

Fixes: ba8f870d

 ("net: dsa: qca8k: add support for mdb_add/del")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>