watchdog: tangox_wdt: test clock rate to avoid division by 0

The clk API may return 0 on clk_get_rate, so we should check the result before
using it as a divisor. For this, refactor the code to use a central
error path.

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