Blackfin Serial Driver: fix bug - use mod_timer to replace only add_timer.

http://blackfin.uclinux.org/gf/project/uclinux-dist/tracker/?action=TrackerItemEdit&tracker_item_id=4045
If adding timer in both timer handler and rx interrupt, a timer may be
added when it is till in the pending list.
Signed-off-by: NSonic Zhang <sonic.zhang@analog.com>
Signed-off-by: NBryan Wu <cooloney@kernel.org>

drivers/serial/bfin_5xx.c

@@ -506,8 +506,7 @@ void bfin_serial_rx_dma_timeout(struct bfin_serial_port *uart)
 		uart->rx_dma_buf.tail = uart->rx_dma_buf.head;
 	}
 
-	uart->rx_dma_timer.expires = jiffies + DMA_RX_FLUSH_JIFFIES;
-	add_timer(&(uart->rx_dma_timer));
+	mod_timer(&(uart->rx_dma_timer), jiffies + DMA_RX_FLUSH_JIFFIES);
 }
 
 static irqreturn_t bfin_serial_dma_tx_int(int irq, void *dev_id)
@@ -550,9 +549,7 @@ static irqreturn_t bfin_serial_dma_rx_int(int irq, void *dev_id)
 	clear_dma_irqstat(uart->rx_dma_channel);
 	spin_unlock(&uart->port.lock);
 
-	del_timer(&(uart->rx_dma_timer));
-	uart->rx_dma_timer.expires = jiffies;
-	add_timer(&(uart->rx_dma_timer));
+	mod_timer(&(uart->rx_dma_timer), jiffies);
 
 	return IRQ_HANDLED;
 }