USB: fix compiler warning fix

This patch (as1123b) fixes a compiler warning: do_unbind_rebind() is
defined but not used if CONFIG_PM=n.

Problem originally found and initial patch submitted by Alexander
Beregalov <a.beregalov@gmail.com>.

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