i40e: enable descriptor prefetch for VFs

As recommended by the hardware guys, enable descriptor prefetch for
rings belonging to VFs. This matches a change already made for ring
belonging to the PF.

Change-ID: Idcc4dc7064bc9144ea81f5701ad07b7ecd11ba49
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>