Staging: batman-adv: Ignore debugfs on kernels without debugfs support

All code for debugfs is ignored when the creation of the batman-adv root
directory in debugfs fails. It must also be ignored when the
debugfs_create_dir tells us that debugfs is not implemented to prevent
possible crashes in the future.

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