RDMA/core: Introduce and use ib_setup_port_attrs()

Refactor code for device and port sysfs attributes for reuse.

While at it, rename counter part free function to ib_free_port_attrs.

Also attribute setup sequence is:
(a) port specific init.
(b) device stats alloc/init.

So for cleanup, follow reverse sequence:
(a) device stats dealloc
(b) port specific cleanup

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