usb: dwc3: gadget: handle request->zero

So far, dwc3 has always missed request->zero
handling for every endpoint. Let's implement
that so we can handle cases where transfer must
be finished with a ZLP.

Note that dwc3 is a little special. Even though
we're dealing with a ZLP, we still need a buffer
of wMaxPacketSize bytes; to hide that detail from
every gadget driver, we have a preallocated buffer
of 1024 bytes (biggest bulk size) to use (and
share) among all endpoints.

Reported-by: Ravi B <ravibabu@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>