usb: gadget: function: printer: avoid wrong list handling in printer_write()
When printer_write() calls usb_ep_queue(), a udc driver (e.g.
renesas_usbhs driver) may call usb_gadget_giveback_request() in
the udc .queue ops immediately. Then, printer_write() calls
list_add(&req->list, &dev->tx_reqs_active) wrongly. After that,
if we do unbind the printer driver, WARN_ON() happens in
printer_func_unbind() because the list entry is not removed.
So, this patch moves list_add(&req->list, &dev->tx_reqs_active)
calling before usb_ep_queue().