net: phylink: constify fwnode arguments

Both phylink_create() and phylink_fwnode_phy_connect() do not modify
the fwnode argument that they are passed, so lets constify these.

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