net: sfp: constify sfp-bus internal fwnode uses

Constify sfp-bus internal fwnode uses, since we do not modify the
fwnode structures.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>