usb: dwc3: gadget: Wait for transfer completion

If a transfer is in-progress, any new request should not kick off
another transfer. The driver needs to wait for the current transfer to
complete before starting off the next transfer. Introduce a new flag
DWC3_EP_WAIT_TRANSFER_COMPLETE for this.

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