usb: ulpi: Fix viewport_addr type

viewport_addr is address of memory mapped ULPI viewport.
It is used only as argument to readl/writel later
causing compile warnings on 64-bit devices.

This fix changes its type to match pointer size.

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Acked-by: Marek Vasut <marex@denx.de>