watchdog: davinci_wdt: fix error handling in davinci_wdt_probe()

clk_disable_unprepare() was added to one error path,
but there is another one. The patch makes sure clk is
disabled at the both of them.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
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>