watchdog: renesas_wdt: avoid (theoretical) type overflow

Because the smallest clock divider we can select is 1, 'clks_per_sec'
must be the same type as 'rate'.

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>