usb: dwc3: omap: use device_for_each_child to handle child removal

Used device_for_each_child() to handle child device (dwc3 core) removal
during devexit of dwc3 omap. This is in preparation for creating the child
devices from subnode of dwc3 omap glue using of_platform_populate.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>