i40e: workaround for XL710 performance

i40e: workaround for XL710 performance

On XL710, performance number is far from the expectation on recent
firmware versions, if promiscuous mode is disabled, or promiscuous
mode is enabled and port MAC address is equal to the packet
destination MAC address. The fix for this issue may not be
integrated in the following firmware version. So the workaround in
software driver is needed. For XL710, it needs to modify the initial
values of 3 internal only registers, which are the same as X710.
Note that the values for X710 and XL710 registers could be different,
and the workaround can be removed when it is fixed in firmware in
the future.

Test report: http://www.dpdk.org/ml/archives/dev/2015-February/012749.html

Signed-off-by: Helin Zhang <helin.zhang@intel.com>
Acked-by: Jingjing Wu <jingjing.wu@intel.com>
Tested-by: Qian Xu <qian.q.xu@intel.com>