watchdog: sa1100: make variable sa1100dog_driver static

This symbol is not used outside of sa1100_wdt.c, so marks it static.

Fixes the following warning:

>> drivers/watchdog/sa1100_wdt.c:241:24: sparse: sparse: symbol 'sa1100dog_driver'
was not declared. Should it be static?

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: sunliming <sunliming@kylinos.cn>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20220802020819.1226454-1-sunliming@kylinos.cn


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