sfc: Move and rename efx_vf struct to siena_vf

The efx_vf struct contains Siena-specific fields for VFs,
so rename to siena_vf.
Also move it into the siena_nic_data struct, as EF10 will
track its VFs in its own ef10_nic_data, storing much less
information about them since VFDI is no longer used.

Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>