mac802154/iface: remove superfluous WARN_ON call in slave_open()

This call was used before we aligned our code with the wireless code base. We
are wanted to handle this in the err: code path. Which would actually not work
because the WARN_ON() macro would reset the res value to 0 and thus we would
never hit err:. Removing it makes the code do what we actually intend.

Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>