USB: use the device lock for persist_enabled

This patch (as1325) changes the locking for the persist_enabled flag
in struct usb_device.  Now it is protected by the device lock, along
with all its neighboring bit flags, instead of the PM lock (which is
about to vanish anyway).

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>