Fix DMA rx ring buffer handling

Reported-by: Qian Zhang <zhangq@sansitech.com>
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/serial/bfin_5xx.c

@@ -401,9 +401,11 @@ static void bfin_serial_dma_rx_chars(struct bfin_serial_port *uart)
 	else
 		flg = TTY_NORMAL;
 
-	for (i = uart->rx_dma_buf.tail; i != uart->rx_dma_buf.head; i++) {
+	for (i = uart->rx_dma_buf.tail; ; i++) {
 		if (i >= UART_XMIT_SIZE)
 			i = 0;
+		if (i == uart->rx_dma_buf.head)
+			break;
 		if (!uart_handle_sysrq_char(&uart->port, uart->rx_dma_buf.buf[i]))
 			uart_insert_char(&uart->port, status, OE,
 				uart->rx_dma_buf.buf[i], flg);