[SERIAL] sunzilog: Fix bugs in device deregristration.

1) Need to unregister 2 ports per of_device.
2) Need to of_iounmap() 1 mapping per of_device.
3) Need to free up the IRQ only after all devices
   have been unregistered.

Signed-off-by: David S. Miller <davem@davemloft.net>