ntb: Remove error checking for debugfs_create_dir()

It is expected that most callers should _ignore_ the errors
return by debugfs_create_dir() in tool_setup_dbgfs()

Signed-off-by: Wang Ming <machel@vivo.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>