trace.c: mtrace_event: fix memcpy_s( dest_size ) argument

Found by chance while looking for something else.

This makes no difference right now because we checked the "length" in
the line immediately before but let's do the right thing anyway, at the
very least it's more readable.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

src/trace/trace.c

@@ -105,7 +105,7 @@ static inline void mtrace_event(const char *data, uint32_t length)
 		trace->pos = 0;
 	}
 
-	memcpy_s(t + trace->pos, MAILBOX_TRACE_SIZE,
+	memcpy_s(t + trace->pos, MAILBOX_TRACE_SIZE - trace->pos,
 		 data, length);
 	dcache_writeback_region(t + trace->pos, length);
 	trace->pos += length;