dtivers: ntb: fix parameter check in perf_setup_dbgfs()

Make IS_ERR() judge the debugfs_create_dir() function return
in perf_setup_dbgfs().

Signed-off-by: Minjie Du <duminjie@vivo.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>