USB: create attributes before sending uevent

This patch (as1087d) fixes a long-standing problem in usbcore: Device,
interface, and endpoint attributes aren't added until _after_ the
creation uevent has already been broadcast.

Unfortunately there are a few attributes which cannot be created that
early.  The "descriptors" attribute is binary and so must be created
separately.  The power-management attributes can't be created until
the dev/power/ group exists.  And the interface string can vary from
one altsetting to another, so it has to be created dynamically.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>