Staging: batman-adv: Call unregister_netdev on failures to get rtnl lock

We must call unregister_netdev when we couldn't initialise the
batman-adv module and the soft_device was registered. There are two
version of the function which we can use:
 * unregister_netdevice - removes device
 * unregister_netdev - takes rtnl semaphore and remove device

We don't hold the semaphore in an error situation. So we must use
unregister_netdev.

Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>