i40e: workaround for X710 performance

i40e: workaround for X710 performance

On X710, performance number is far from the expectation on recent
firmware versions. The fix for this issue may not be integrated in
the following firmware version. So the workaround in software driver
is needed. It needs to modify the initial values of 3 internal only
registers. Note that the workaround can be removed when it is fixed
in firmware in the future.

Signed-off-by: Helin Zhang <helin.zhang@intel.com>
Acked-by: Jing Chen <jing.d.chen@intel.com>