usb-uas: hotplug support

Make attached property settable and turns off auto-attach in case the
device was hotplugged.  Hotplugging works simliar to usb-bot now.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1465984019-28963-6-git-send-email-kraxel@redhat.com