realtek: pcs: store SerDes fwnode instead of device_node

realtek: pcs: store SerDes fwnode instead of device_node

Switch rtpcs_serdes from struct device_node * to struct fwnode_handle *
in preparation for fwnode_pcs_add_provider, which keys providers by
fwnode. Storing the fwnode directly avoids of_fwnode_handle() wrappers
at every API boundary.

The conversion is mechanical: of_node_get/put become fwnode_handle_get/
put (same refcount on OF-backed fwnodes), polarity helpers drop their
of_fwnode_handle() wrapping, and the link counter compares fwnodes
directly via of_fwnode_handle(arg_np). No behavior change.

Link: https://github.com/openwrt/openwrt/pull/23539
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>