i40evf: Use macro param for ethtool stats

Use a macro parameter for ethtool stats instead of just assuming
that a valid netdev variable exists. Suggested by Ben Hutchings.

CC: Ben Hutchings <ben@decadent.org.uk>
Change-ID: I66681698573c1549f95fdea310149d8a7e96a60f
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>