dm: usb: Make usb_get_bus easier to use for callers

dm: usb: Make usb_get_bus easier to use for callers

Make usb_get_bus easier to use for callers, by directly returning the bus
rather then returning it via a pass-by-ref argument.

This also removes the error checking from the current callers, as
we already have an assert() for bus not being NULL in usb_get_bus().

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Simon Glass <sjg@chromium.org>