watchdog: kempld_wdt: Fix bit mask definition

STAGE_CFG bits are defined as [5:4] bits. However, '(((x) & 0x30) << 4)'
handles [9:8] bits. Thus, it should be fixed in order to handle
[5:4] bits.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>