RDMA/cxgb4: Detach from the LLD after unregistering RDMA device

In the RDMA core unregister path, kernel users will be calling down
into the T4 provider to release resources.  So we cannot detach from
the LLD until this process completes.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>