usb/core/driver: fix warning

usb/core/driver: fix warning:
drivers/usb/core/driver.c:834: warning: 'do_unbind_rebind' defined but not used

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>