test: dm: skip scmi tests against disabled protocols

This is a precautionary change to make scmi tests workable whether or not
a specific protocol be enabled. If a given protocol is not configured,
we skip the test by returning -EAGAIN.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>