hw/rdma: Do not call rdma_backend_del_gid on an empty gid

hw/rdma: Do not call rdma_backend_del_gid on an empty gid

When device goes down the function fini_ports loops over all entries in
gid table regardless of the fact whether entry is valid or not. In case
that entry is not valid we'd like to skip from any further processing in
backend device.

Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>