phy: bcm-ns-usb3: use pointer for PHY writing function

Our current writing function accesses PHY directly bypassing MDIO layer.

The aim is to extend this module to also behave as MDIO driver. This
will require using different writing function which can be handled
cleanly by having an extra pointer like this.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>