i40e: Fix error handling in i40e_init_module()

i40e_init_module() won't free the debugfs directory created by
i40e_dbg_init() when pci_register_driver() failed. Add fail path to
call i40e_dbg_exit() to remove the debugfs entries to prevent the bug.

i40e: Intel(R) Ethernet Connection XL710 Network Driver
i40e: Copyright (c) 2013 - 2019 Intel Corporation.
debugfs: Directory 'i40e' with parent '/' already present!

Fixes: 41c445ff

 ("i40e: main driver core")
Signed-off-by: Shang XiaoJing <shangxiaojing@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Tested-by: Gurucharan G <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>