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>