usb: gadget: f_tcm: Inform last stream request

Set the request->is_last to each stream request to indicate that the
request is the last stream request of a transfer. The DWC3 controller
needs to know this info to properly switch streams. The current
implementation of f_tcm uses a single request per transfer, so every
stream request is the last of its stream.

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