RDMA/core: Fix return code when modify_device isn't supported

The proper return code is "-EOPNOTSUPP" when modify_device callback is not
supported.

Link: https://lore.kernel.org/r/20190923104158.5331-2-kamalheib1@gmail.com


Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>