watchdog: max63xx: make module's license marker match the header

The header specifies GPL version 2 only, so make the MODULE_LICENSE
string use the respective string for that.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>