usb: dwc3: gadget: Don't prepare beyond a transfer

Don't prepare TRBs beyond a transfer. In DWC_usb32, its transfer burst
capability may try to read and use TRBs beyond the active transfer. For
other controllers, they don't process the next transfer TRBs until the
current transfer is completed. Explicitly prevent preparing TRBs ahead
for all controllers.

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>