net/i40e: fix link down and negotiation

net/i40e: fix link down and negotiation

Enable the functions set link down and set link up in i40e by check
phy_type, and fix the issue of auto negotiation failed in XXV710 when
bind kernel driver after unbind from DPDK driver by modify the speed
setting distinguish from set link up and down. With this fix, if unbind
DPDK to bind kernel driver, no need to set auto negotiation and ifconfig
up anymore, remove the part from doc.

Fixes: ca7e599d4506 ("net/i40e: fix link management")
Fixes: 2f1e22817420 ("i40e: skip link control as firmware workaround")
Fixes: 6e145fcc754b ("i40e: support autoneg or force link speed")
Cc: stable@dpdk.org
Signed-off-by: Jeff Guo <jia.guo@intel.com>
Acked-by: Jingjing Wu <jingjing.wu@intel.com>