netdevsim: delete unnecessary debugfs checking

In normal situations where the driver doesn't dereference
"nsim_node->ddir" or "nsim_node->rate_parent" itself then we are not
supposed to check the return from debugfs functions.  In the case of
debugfs_create_dir() the check was wrong as well because it doesn't
return NULL, it returns error pointers.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>