chore(epicardium-api): Fix sign-compare warnings

Fix all warnings found by `-Wsign-compare` in epicardium/api.

Signed-off-by: Rahix <rahix@rahix.de>

epicardium/api/caller.c

@@ -109,7 +109,7 @@ int api_fetch_args(char *buf, size_t cnt)
 		return 0;
 	}
 
-	int i;
+	size_t i;
 	for (i = 0; i < cnt && API_CALL_MEM->buffer[i + 0x20] != '\0'; i++) {
 		buf[i] = API_CALL_MEM->buffer[i + 0x20];
 	}

epicardium/api/dispatcher.c

@@ -86,7 +86,7 @@ void api_prepare_args(char *args)
 	 * collide with any integer return value of API calls like epic_exec().
 	 */
 	API_CALL_MEM->id = 0;
-	for (int i = 0; i <= strlen(args); i++) {
+	for (size_t i = 0; i <= strlen(args); i++) {
 		API_CALL_MEM->buffer[i + 0x20] = args[i];
 	}
 }

epicardium/api/interrupt-sender.c

@@ -11,7 +11,7 @@ int api_interrupt_trigger(api_int_id_t id)
 	}
 
 	if (int_enabled[id]) {
-		while (API_CALL_MEM->int_id != (-1))
+		while (API_CALL_MEM->int_id != (api_int_id_t)(-1))
 			;
 
 		API_CALL_MEM->int_id = id;