Documentation: sysfs-bus-usb: update power/persist description

There's no power/persist file for hubs. And CONFIG_USB_PERSIST was
removed in v2.6.26. Update the description of power/persist accordingly.
Also remove the line on its default value. It is not entirely correct, as
CONFIG_USB_DEFAULT_PERSIST and the USB_QUIRK_RESET flag influence the
default. It is not needed to understand this file anyhow.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>