sfp: fix sfp-bus oops when removing socket/upstream

When we remove a socket or upstream, and the other side isn't
registered, we dereference a NULL pointer, causing a kernel oops.
Fix this.

Fixes: ce0aa27f ("sfp: add sfp-bus to bridge between network devices and sfp cages")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/sfp-bus.c

@@ -356,6 +356,7 @@ EXPORT_SYMBOL_GPL(sfp_register_upstream);
 void sfp_unregister_upstream(struct sfp_bus *bus)
 {
 	rtnl_lock();
+	if (bus->sfp)
 		sfp_unregister_bus(bus);
 	bus->upstream = NULL;
 	bus->netdev = NULL;
@@ -459,6 +460,7 @@ EXPORT_SYMBOL_GPL(sfp_register_socket);
 void sfp_unregister_socket(struct sfp_bus *bus)
 {
 	rtnl_lock();
+	if (bus->netdev)
 		sfp_unregister_bus(bus);
 	bus->sfp_dev = NULL;
 	bus->sfp = NULL;