watchdog: s3c2410: Simplify getting driver data

Simplify the flow in helper function for getting the driver data by
using of_device_get_match_data() and only one if() branch.

The code should be equivalent.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>