greybus: es2: fix arpc response lookups

Fix arpc response lookups that were truncating the 16-bit response id to
8-bit, something which would have lead to all arpc calls timing out
after the 256th request.

Testing done: Enumerated and ejected a module on EVT2.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Alex Elder <elder@linaro.org>