net: sfp: add mandatory attach/detach methods for sfp buses

Add attach and detach methods for SFP buses, which will allow us to get
rid of the netdev storage in sfp-bus.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>