s390/qdio: simplify debugfs code

There's no need for error handling, the debugfs core is smart enough to
deal with IS_ERR() internally.

This will also keep us from creating the debugfs files if the device
directory doesn't exist. Currently (because irq_ptr->debugfs_dev gets set
to NULL on error) the files would be placed into the debugfs root - without
any association to their parent device.

On teardown, use the debugfs_remove_recursive() helper to avoid keeping
track of each created file/directory.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>