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.

firmware/src_simtrace/mode_cardemu.c

@@ -1,3 +1,5 @@
+#define TRACE_LEVEL 6
+
 #include "board.h"
 #include "simtrace.h"
 #include "ringbuffer.h"
@@ -250,6 +252,9 @@ void mode_cardemu_exit(void)
 {
 	TRACE_ENTRY();
 
+	/* FIXME: stop tc_fdt */
+	/* FIXME: release all rctx, unlink them from any queue */
+
 	PIO_DisableIt(&pin_usim1_rst);
 	PIO_DisableIt(&pin_usim1_vcc);