ntb: intel: Fix the NULL vs IS_ERR() bug for debugfs_create_dir()

The debugfs_create_dir() function returns error pointers.
It never returns NULL. So use IS_ERR() to check it.

Fixes: e26a5843 ("NTB: Split ntb_hw_intel and ntb_transport drivers")
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>