sfc: Split STATE_READY in to STATE_NET_DOWN and STATE_NET_UP.

This patch splits the READY state in to NET_UP and NET_DOWN. This
is to prepare for future work to delay resource allocation until
interface up so that we can use resources more efficiently in
SRIOV environments, and also to lay the ground work for an extra
PROBED state where we don't create a network interface,
for VDPA operation.

Signed-off-by: Jonathan Cooper <jonathan.s.cooper@amd.com>
Acked-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>