sfc: Remove siena_nic_data and stats

These are no longer used, and the two  Siena specific functions are
no longer present in sfc.ko.

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