i40e: Fix sparse errors in i40e_txrx.c

Remove error handling through pointers. Instead use plain int
to return value from i40e_run_xdp(...).

Previously:
- sparse errors were produced during compilation:
i40e_txrx.c:2338 i40e_run_xdp() error: (-2147483647) too low for ERR_PTR
i40e_txrx.c:2558 i40e_clean_rx_irq() error: 'skb' dereferencing possible ERR_PTR()

- sk_buff* was used to return value, but it has never had valid
pointer to sk_buff. Returned value was always int handled as
a pointer.

Fixes: 0c8493d9 ("i40e: add XDP support for pass and drop actions")
Fixes: 2e689312

 ("i40e: split XDP_TX tail and XDP_REDIRECT map flushing")
Signed-off-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Tested-by: Dave Switzer <david.switzer@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>