watchdog: Make watchdog_class const

Now that the driver core allows for struct class to be in read-only
memory, mark watchdog_class as const.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20240614-class-const-wdt-v1-1-f9a4e2b1ba76@weissschuh.net


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>