bridge: mdb: allow the user to delete mdb entry if there's a querier

Until now when a querier was present static entries couldn't be deleted.
Fix this and allow the user to manipulate the mdb with or without a
querier.

Signed-off-by: Satish Ashok <sashok@cumulusnetworks.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>