usb: dwc3: use dev_get_platdata()

Use the wrapper function for retrieving the
platform_data instead of accessing dev->platform_data
directly.

While at that also make change 'node' initialization
to use the dev pointer.

Inspired-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>