watchdog: at91sam9_wdt: fix secs_to_ticks

Fix the secs_to_ticks macro in case 0 is passed as an argument.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>