watchdog: imx2_wdt: Disable previously acquired clock on error path

If watchdog_register_device() fails we should disable the previously
acquired wdev->clk clock on error path.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>