qlcnic: Add VXLAN Rx offload support

This patch adds Rx checksum offload support for VXLAN.
Implements .ndo_{add|del}_vxlan_port netdev ops.
Adapter supports only one VXLAN port, so program adapter with
very first UDP port which VXLAN driver is listening to.

Signed-off-by: Shahed Shaikh <shahed.shaikh@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>