net/ixgbe: fix uninitialized variable

In func ixgbe_dev_link_update(), "link.link_autoneg" is used in func
call rte_ixgbe_dev_atomic_write_link_status(), but is uninitialized.

Fixes: 82113036e4e5 ("ethdev: redesign link speed config")
Cc: stable@dpdk.org
Signed-off-by: Yong Wang <wang.yong19@zte.com.cn>
Acked-by: Wenzhuo Lu <wenzhuo.lu@intel.com>