i40e: Fix an NULL vs IS_ERR() bug for debugfs_create_dir()

The debugfs_create_dir() function returns error pointers.
It never returns NULL. Most incorrect error checks were fixed,
but the one in i40e_dbg_init() was forgotten.

Fix the remaining error check.

Fixes: 02e9c290

 ("i40e: debugfs interface")
Signed-off-by: Wang Ming <machel@vivo.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>