RDMA/cxgb4: Register RDMA provider based on LLD state_change events

The LLD now supports proper UP state change events, so move the RDMA
provider registration to UP path.

This fixes a crash when loading iw_cxgb4 _after_ the NFS/RDMA
transport is up and running.

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