RDMA/core: Depend on device_add() to add device attributes

Instead of adding/removing device attribute files, depend on device_add()
which considers adding these device files based on NULL terminated
attributes group array.

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