watchdog: Fix rio watchdog probe function

After sucessfully registering the misc device the driver iounmaps the
hardware registers and kfree's the device data structure. Ouch !

This was introduced with commit e42311d7

 (riowatchdog: Convert to
pure OF driver) and went unnoticed for more than a year :)

Return success instead of dropping into the error cleanup code path.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>