When unregister gadget driver in ci_udc, the usb device is not
removed or stop. This causes next "usb start" fails to work.
Add a new interface "usb_remove_ehci_gadget" in usb-uclass to
remove the usb device for DM driver. Using "usb_lowlevel_stop" for
non-DM driver.
Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>