serial: sh-sci: Fix cast warning

Fix the following compile warning about cast to pointer from
integer of different size.

drivers/tty/serial/sh-sci.c:2021:19: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>