sfc: Remove vfdi.h

It was only used for Siena SRIOV, so nothing includes it any more
in this directory.

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>