i40e: Fix a potential NULL pointer dereference

If 'kzalloc()' fails, a NULL pointer will be dereferenced.
Return an error code (-ENOMEM) instead.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>