Performance can be improved a bit by imitating ixgbe and using
prefetch to get us the next Tx descriptor.

Change-ID: Ice7ffd4cd0ce87c35295059bdb7972a7f53723aa
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>