serial: samsung: introduce s3c24xx_serial_rx_drain_fifo() function

This patch introduces s3c24xx_serial_rx_drain_fifo() which reads data
from RX FIFO and writes it to tty buffer. It also checks for special
conditions (such as 'break') and handles it. This function has been
separated from s3c24xx_serial_rx_chars_pio() as it contains code which
can be used also in DMA mode.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>