sfc: Add a PROBED state for EF100 VDPA use.

For VDPA we need to tear down the driver to the point where it
has various control channels like MCDI, but it no longer has
a network device. This adds a state corresponding to
that mode that will be used when VDPA support is added.

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>