i40evf: make ethtool_ops const

Const-ify the ethtool_ops structure, as it is extremely unlikely to
change at runtime. Suggested by Ben Hutchings.

CC: Ben Hutchings <ben@decadent.org.uk>
Change-ID: I1ccb1b7c3ea801cc934447599a35910e7c93d321
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>