[S390] qdio: fix broken pointer in case of CONFIG_DEBUG_FS is disabled

If CONFIG_DEBUG_FS is disabled debugfs_create_file returns -ENODEV instead
of zero. Since the return value is stored and used at shutdown to remove
existing entries an OOPS may be triggered.

Add a check of the debugfs_create_file return value and in case of an error
set the entry to NULL so it will be ignored at shutdown.

Signed-off-by: Jan Glauber <jang@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>