ptp: use is_visible method to hide unused attributes

Instead of creating selected attributes after the device is created (and
after userspace potentially seen uevent), lets use attribute group
is_visible() method to control which attributes are shown. This will allow
us to create all attributes (except "pins" group, which will be taken care
of later) before userspace gets notified about new ptp class device.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>