watchdog: sb_wdog: Make sbwdog_set and sbwdog_pet static

Fix sparse warning:

drivers/watchdog/sb_wdog.c:70:6: warning: symbol 'sbwdog_set' was not declared. Should it be static?
drivers/watchdog/sb_wdog.c:84:6: warning: symbol 'sbwdog_pet' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Mukesh Ojha <mojha@codeaurora.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@linux-watchdog.org>