i40e: Fix potential invalid access when MAC list is empty

list_first_entry() never returns NULL - if the list is empty, it still
returns a pointer to an invalid object, leading to potential invalid
memory access when dereferenced.

Fix this by using list_first_entry_or_null instead of list_first_entry.

Fixes: e3219ce6 ("i40e: Add support for client interface for IWARP driver")
Signed-off-by: Zhen Ni <zhen.ni@easystack.cn>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>