usb: dwc3: gadget: Delay starting transfer

If the END_TRANSFER command hasn't completed yet, then don't send the
START_TRANSFER command. The controller may not be able to start if
that's the case. Some controller revisions depend on this. See
commit 76a638f8 ("usb: dwc3: gadget: wait for End Transfer to
complete"). Let's only send START_TRANSFER command after the
END_TRANSFER command had completed.

Fixes: 3aec9915 ("usb: dwc3: gadget: remove DWC3_EP_END_TRANSFER_PENDING")
Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>