i40evf: Use an iterator of the same type as the list

When iterating through the linked list of VLAN filters, make the
iterator the same type as that of the linked list.

Signed-off-by: Harshitha Ramamurthy <harshitha.ramamurthy@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>