net: sfp: fix unbind

When unbinding, we don't correctly tear down the module state, leaving
(for example) the hwmon registration behind. Ensure everything is
properly removed by sending a remove event at unbind.

Fixes: 6b0da5c9 ("net: sfp: track upstream's attachment state in state machine")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>