The ports kobject isn't being released during error flow in device
registration.  This patch refactors the ports kobject cleanup into a
single function called from both the error flow in device registration
and from the unregistration function.

A couple of attributes aren't being deleted (iw_stats_group, and
ib_class_attributes).  While this may be handled implicitly by the
destruction of their kobjects, it seems better to handle all the
attributes the same way.

Signed-off-by: Haggai Eran <haggaie@mellanox.com>

[ Make free_port_list_attributes() static.  - Roland ]

Signed-off-by: Roland Dreier <roland@purestorage.com>