sfp: convert to fwnode

Convert sfp-bus to use fwnode rather than device_node internally, so
we can support more than just device tree firmware.

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>