i40e/virtchnl: fix application of sizeof to pointer

sizeof when applied to a pointer typed expression gives the size of
the pointer.

The proper fix in this particular case is to code sizeof(*vfres)
instead of sizeof(vfres).

This issue was detected with the help of Coccinelle.

Signed-off-by: Gustavo A R Silva <garsilva@embeddedor.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>