serial: bfin_uart: narrow the reboot condition in DMA tx interrupt

Check if xmit buffer pointers are set to zero.
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/bfin_uart.c

@@ -532,7 +532,7 @@ static irqreturn_t bfin_serial_dma_tx_int(int irq, void *dev_id)
 		 */
 		UART_CLEAR_IER(uart, ETBEI);
 		uart->port.icount.tx += uart->tx_count;
-		if (!uart_circ_empty(xmit)) {
+		if (!(xmit->tail == 0 && xmit->head == 0)) {
 			xmit->tail = (xmit->tail + uart->tx_count) & (UART_XMIT_SIZE - 1);
 
 			if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)