fix(scmi-msg): base: fix protocol list querying

Corrects the function reporting the SCMI protocols supported by the
platform to not assume 8 protocol IDs at most can be returned. Indeed
the number of protocol IDs returned depends on the SCMI output buffer
size.

Change-Id: Idafbe02d2b25b3bcacaf25977c560c0ac5bb8d62
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>