Blackfin Serial Driver: disable dma rx interrupt only rather than all irqs

The UART RX handling code isn't exactly speeding, so don't go disabling
all interrupts when processing the buffer.  Just disable the relevant DMA
interrupt.  This greatly improves latency of the system when utilizing the
UART.
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/serial/bfin_5xx.c

@@ -479,9 +479,9 @@ static void bfin_serial_dma_rx_chars(struct bfin_serial_port *uart)
 void bfin_serial_rx_dma_timeout(struct bfin_serial_port *uart)
 {
 	int x_pos, pos;
-	unsigned long flags;
 
-	spin_lock_irqsave(&uart->port.lock, flags);
+	dma_disable_irq(uart->rx_dma_channel);
+	spin_lock_bh(&uart->port.lock);
 
 	/* 2D DMA RX buffer ring is used. Because curr_y_count and
 	 * curr_x_count can't be read as an atomic operation,
@@ -512,7 +512,8 @@ void bfin_serial_rx_dma_timeout(struct bfin_serial_port *uart)
 		uart->rx_dma_buf.tail = uart->rx_dma_buf.head;
 	}
 
-	spin_unlock_irqrestore(&uart->port.lock, flags);
+	spin_unlock_bh(&uart->port.lock);
+	dma_enable_irq(uart->rx_dma_channel);
 
 	mod_timer(&(uart->rx_dma_timer), jiffies + DMA_RX_FLUSH_JIFFIES);
 }