usb: ehci: mxs: Fix the USB node pointer retrieval

Use dev_ofnode() to retrieve the USB node pointer from the udevice
structure.

This fixes the following build error:

drivers/usb/host/ehci-mxs.c:143:38: error: 'struct udevice' has no member named 'node_'

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>