sfc: Encapsulate access to netdev_priv()

Once we separate struct efx_nic memory from net_device memory the
existing usage will have to change.
Apart from the new function efx_netdev_priv() accesses have been
changed using:
  sed -i 's/netdev_priv/efx_netdev_priv/'

Signed-off-by: Jonathan Cooper <jonathan.s.cooper@amd.com>
Co-developed-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>