bi2a: Clarify data field usage on init command

The single byte is a specific parameter required to initialize
the IO correctly. Otherwise, it either will only work partially
or not worked at all. The full impact and meaning of that byte
is not entirely known, yet.

src/main/acio/acio.h

@@ -66,6 +66,7 @@ struct ac_io_message {
                 uint8_t raw[0xFF];
                 uint8_t count;
                 uint8_t status;
+                uint8_t param;
                 struct ac_io_version version;
 
                 struct ac_io_icca_misc icca_misc;