intel/i40e: delete if NULL check before dev_kfree_skb

dev_kfree_skb check if the input parameter NULL and do the right
thing, there is no need to check again.
This change is to cleanup the code a bit.

Signed-off-by: Bernard Zhao <zhaojunkui2008@126.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>