i40evf: get rid of pci_using_dac

PCI DAC doesn't really mean much on a virtualized PCI Express part, so
get rid of that check and just always set the HIGHDMA flag in the net
device.

Change-ID: I2040272be0e7934323f470c2bc73fbdd4f93e2b6
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>