winverbs: fix event handler crash

Winverbs registers for HCA events using the WV_RDMA_DEVICE object,
rather than the WV_DEVICE object.  The latter corresponds to a user's
opened device and is where HCA events should be reported.

This fixes a crash in winverbs when reporting a port down event.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>