watchdog: s3c2410_wdt: Fix the mask bit offset for Exynos7

The watchdog mask bit offset listed for Exynos7 is incorrect.
Fix this.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Acked-by: Naveen Krishna Chatradhi <naveenkrishna.ch@gmail.com>
Reviewd-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>