i40evf: enforce descriptor write-back mechanism for VF

The current driver mode is to use a write-back mechanism for the head
register which indicates transmit completions. The VF driver needs to be
able to work on hardware that exclusively uses descriptor write-back, so
change the default driver mode of operation to descriptor write-back for
VF. In our analysis, performance wasn't significantly different with
either write-back method.

Change-ID: Ia92e4ec77c2df8dc4515c71d53746d57d77759af
Signed-off-by: Preethi Banala <preethi.banala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>