IB/hns: Register HNS RoCE Driver get_netdev() with IB Core

This patch adds get_netdev() function to the IB device. This shall be
used to fetch netdev corresponding to the port number. This function
would be called by IB core(Generic CM Agent) for example, when the
RDMA connection is being established.

Signed-off-by: Lijun Ou <oulijun@huawei.com>
Reviewed-by: Wei Hu <xavier.huwei@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>