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: NMitch Williams <mitch.a.williams@intel.com>
Tested-by: NSibai Li <sibai.li@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>