usb: musb: Silence error about blacklisting hubs if !CONFIG_USB

Some drivers, like jz4740-musb, don't depend on CONFIG_USB.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>