sfc: mark state UNINIT after unregister

Without this change, modprobe -r sfc hits the BUG_ON() in
efx_pci_remove_main().

Fixes: e7fef9b4 ("sfc: add sysfs entry to control MCDI tracing")
Reported-by: Jarod Wilson <jarod@redhat.com>
Reviewed-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>