RDMA: Use ib_gid_attr during GID modification

Now that ib_gid_attr contains device, port and index, simplify the
provider APIs add_gid() and del_gid() to use device, port and index
fields from the ib_gid_attr attributes structure.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>