scsi: qla2xxx: Fix NULL vs IS_ERR() bug for debugfs_create_dir()

Since both debugfs_create_dir() and debugfs_create_file() return ERR_PTR
and never NULL, use IS_ERR() instead of checking for NULL.

Fixes: 1e98fb0f

 ("scsi: qla2xxx: Setup debugfs entries for remote ports")
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Link: https://lore.kernel.org/r/20230831140930.3166359-1-ruanjinjie@huawei.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>