IB/usnic: Advertise usNIC devices as RDMA_NODE_USNIC_UDP

usNIC default transport is UDP.  Hence, advertise RDMA_NODE_USNIC_UDP
by default for usNIC devices.

Signed-off-by: Upinder Malhi <umalhi@cisco.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>