dmatest: remove IS_ERR_OR_NULL checks of debugfs calls

There is a really little chance when we are able to create a directory and are
not able to create nodes under it. So, this patch just removes those checks.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dan Williams <djbw@fb.com>