RDMA/core: make ib_device.add method optional

ib_clients can indeed fill .add to NULL, but then they will not see
any device removal notifications. The reason is that that
ib_register_client and ib_register_device checked existence of .add
before adding the creating a corresponding client_data and adding
it to the list. Simple condition reverse fixes the issue.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>