fix parted of "missing for discarded return value"

MISRA C required that return value should be used, missing for it should
add "(void)" prefix before the function call.
Some function can be declared without return value to avoid this problem.

Signed-off-by: Huihuang Shi <huihuang.shi@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>

hypervisor/debug/vuart.c

@@ -61,7 +61,7 @@ static void fifo_init(struct fifo *fifo, int sz)
 	fifo_reset(fifo);
 }
 
-static char fifo_putchar(struct fifo *fifo, char ch)
+static void fifo_putchar(struct fifo *fifo, char ch)
 {
 	fifo->buf[fifo->windex] = ch;
 	if (fifo->num < fifo->size) {
@@ -71,7 +71,6 @@ static char fifo_putchar(struct fifo *fifo, char ch)
 		fifo->rindex = (fifo->rindex + 1) % fifo->size;
 		fifo->windex = (fifo->windex + 1) % fifo->size;
 	}
-	return 0;
 }
 
 static char fifo_getchar(struct fifo *fifo)