This allows the endpoints to work when multiple MUSB glue
layers are built in.

Cc: Fabio Baltieri <fabio.baltieri@linaro.org>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Apelete Seketeli <apelete@seketeli.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>