serial: samsung: add support for manual RTS setting

The Samsung serial driver currently does not support setting the
RTS pin with an ioctl(TIOCMSET) call. This patch adds this support.

Changes in v2:
 - Preserve the RTS pin's manual setting  in set_termios() also when
   enabling CRTSCTS.

Signed-off-by: José Miguel Gonçalves <jose.goncalves@inov.pt>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>