usb: dwc3: gadget: return early in dwc3_cleanup_done_reqs()

This patch avoids the compiler spitting out the following warning:
|drivers/usb/dwc3/gadget.c:1304: warning: 'trb' is used uninitialized \
	in this function

This is only uninitialized if the list of to-cleanup TRBs is empty which
should not be the case because we call this functions once a transfer
completed so it should be on list.

In order to make the warning disappear we return early. This should
never happen and the WARN_ON_ONCE(1) is there in case it happens
so we can investigate what went wrong.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>