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>