bonding: remove redundant NULL check in debugfs function

Because debugfs_create_dir returns ERR_PTR, so bonding_debug_root will
never be NULL. Remove redundant NULL check for bonding_debug_root in
debugfs function. The later debugfs_create_dir/debugfs_remove_recursive
/debugfs_remove_recursive functions will check the dentry with IS_ERR().

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>