ringbuffer: Make ring buffer access irq-save

We have to temporarily disable nterrupts when performing rinbuffer
operations, as the ring buffer is used both from IRQ as well as process
context.
diff --git a/firmware/src_simtrace/card_emu.c b/firmware/src_simtrace/card_emu.c
index dd0899f..8376943 100644
--- a/firmware/src_simtrace/card_emu.c
+++ b/firmware/src_simtrace/card_emu.c
@@ -17,6 +17,8 @@
  *
  */
 
+#define TRACE_LEVEL 6
+
 #include <assert.h>
 #include <errno.h>
 #include <string.h>