serial: s5p: set automatically clears after resetting Rx FIFO

This patch fix the u-boot shell problem on TRATS2 board.
 - If hold the key while booting is in progress,
   white spaces are written in u-boot shell.

Set Automatically clears after resetting Rx FIFO.

Signed-off-by: Inha Song <ideal.song@samsung.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>

drivers/serial/serial_s5p.c

@@ -100,8 +100,8 @@ static int serial_init_dev(const int dev_index)
 {
 	struct s5p_uart *const uart = s5p_get_base_uart(dev_index);
 
-	/* enable FIFOs */
-	writel(0x1, &uart->ufcon);
+	/* enable FIFOs, auto clear Rx FIFO */
+	writel(0x3, &uart->ufcon);
 	writel(0, &uart->umcon);
 	/* 8N1 */
 	writel(0x3, &uart->ulcon);