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>

drivers/net/ethernet/intel/i40e/i40e_debugfs.c

@@ -1839,7 +1839,7 @@ void i40e_dbg_pf_exit(struct i40e_pf *pf)
 void i40e_dbg_init(void)
 {
 	i40e_dbg_root = debugfs_create_dir(i40e_driver_name, NULL);
-	if (!i40e_dbg_root)
+	if (IS_ERR(i40e_dbg_root))
 		pr_info("init of debugfs failed\n");
 }