sfc: Disable Siena support

Disable the build of Siena code until later in this patch series.
Prevent sfc.ko from binding to Siena NICs.

efx_init_sriov/efx_fini_sriov is only used for Siena. Remove calls
to those.

Signed-off-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>