macsec: Use helper macsec_netdev_priv for offload drivers

Now macsec on top of vlan can be offloaded to macsec offloading
devices so that VLAN tag is sent in clear text on wire i.e,
packet structure is DMAC|SMAC|VLAN|SECTAG. Offloading devices can
simply enable NETIF_F_HW_MACSEC feature in netdev->vlan_features for
this to work. But the logic in offloading drivers to retrieve the
private structure from netdev needs to be changed to check whether
the netdev received is real device or a vlan device and get private
structure accordingly. This patch changes the offloading drivers to
use helper macsec_netdev_priv instead of netdev_priv.

Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>