i40e: future proof some sizeof calls

Make sure the sizeof() calls are taking the size of the actual struct
that we care about.  By using the pointer variable, we'll always get
the right struct size, even if the variable type changes sometime in
the future.

Change-ID: Id5858f883cf42447365ea3733080d7714f975bce
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>