net: sfp: make sfp_bus_find_fwnode() take a const fwnode

sfp_bus_find_fwnode() does not write to the fwnode, so let's make it
const.

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>