net: macsec: allow to reference a netdev from a MACsec context

This patch allows to reference a net_device from a MACsec context. This
is needed to allow implementing MACsec operations in net device drivers.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: Mark Starovoytov <mstarovoitov@marvell.com>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>