Revert "usb: udc: allow adding and removing the same gadget device"

We have already allocated gadget structure dynamically at UDC (dwc3)
driver, so commit fac32347

 ("usb: udc: allow adding and removing
the same gadget device")could be reverted.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>