bootstd: usb: Avoid initing USB twice

This causes crashes on some boards, e.g. rockpro64. In any case, we
should not do it.

Check the usb_started flag to avoid this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Jonas Karlman <jonas@kwiboo.se>
Tested-by: Tom Rini <trini@konsulko.com>