dm: usb: Rename usb_find_child to usb_find_emul_child

Now that we unbind usb devices from usb_stop() usb_find_child() is
only necessary to deal with emulated usb devices.

Rename it to make this clear and add a #ifdef to make it a nop in
other cases.

Note the #ifdef turns usb_find_emul_child() into a nop, rather then not
building it and adding another #ifdef to the caller, this is done this way
because adding a #ifdef to the caller is somewhat hairy.

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