shrink ringbuffer size to 128 bytes

We only use it to implemet a software FIFO between the UART Rx IRQ
and the code that processes it.  1k is way too large for that.

firmware/src_simtrace/ringbuffer.h

@@ -5,7 +5,7 @@
 #include <stdbool.h>
 #include <sys/types.h>
 
-#define RING_BUFLEN 1024
+#define RING_BUFLEN 128
 
 typedef struct ringbuf {
 	uint8_t buf[RING_BUFLEN];