watchdog: renesas_wdt: check rate also for upper limit

When checking the clock rate, ensure also that counting all 16 bits
takes at least one second to match the granularity of the framework.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>