dm: usb: ehci: Use a function to find the controller from struct udevice

dm: usb: ehci: Use a function to find the controller from struct udevice

With driver model we want to remove the controller pointer in struct udevice
and use driver model data structures instead. To prepare for this, move
access to this field to a function which can provide a different
implementation for driver model.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>