ixgbevf: make ethtool ops and strings const

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>