Make integer to enum conversion explicit [SAR-261]

[fpezzinosn]

The signature of the function sbp_register_callback_generic defined here expects an sbp_msg_type_t enum as a second parameter. The same happens with the third parameter of process_frame defined here.

Without this explicit conversion, the static analyzer thinks there are two different functions with the same name.

[silverjam]

Need to update the templates, otherwise these changes will be lost when we add new messages: https://github.com/swift-nav/libsbp/blob/master/generator/sbpg/targets/resources/c/sbp_msg_type_template.h#L33