netdevsim: remove dir in nsim_dev_debugfs_init() when creating ports dir failed

Remove dir in nsim_dev_debugfs_init() when creating ports dir failed.
Otherwise, the netdevsim device will not be created next time. Kernel
reports an error: debugfs: Directory 'netdevsim1' with parent 'netdevsim'
already present!

Fixes: ab1d0cc0

 ("netdevsim: change debugfs tree topology")
Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>