treewide: use C enum types explicitly

treewide: use C enum types explicitly

Use enum types explicitly across the entire C API. Modern compilers can
catch some bugs with enums and it also helps IDEs and general readability.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
[Viresh: make rust bindings work with negative enum values]
Co-authored-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>