watchdog: mpc8xxx_wdt: Constify static struct watchdog_ops

watchdog: mpc8xxx_wdt: Constify static struct watchdog_ops

The struct mpc8xxx_wdt_ops is only assigned to the ops pointer in the
watchdog_device struct, which is a pointer to const struct watchdog_ops.
Make it const to allow the compiler to put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210727223042.48150-4-rikard.falkeborn@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>