watchdog: bcm63xx_wdt: fix fallthrough warning

This fixes:
drivers/watchdog/bcm63xx_wdt.c: In function 'bcm63xx_wdt_ioctl':
drivers/watchdog/bcm63xx_wdt.c:208:17: warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20211027123135.27458-1-zajec5@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>