USB: gadget: g_serial: append zlp when tx buffer becomes empty

Some usb serial host drivers expect a short packet before they forward
the data to the application. This is caused by them trying to read more
than one packet at a time. So when the gadget sends an exact multiple
of the maximum packet size, it should append a zero-length packet.
Signed-off-by: NDaniel Glöckner <dg@emlix.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/gadget/u_serial.c

@@ -371,6 +371,7 @@ __acquires(&port->port_lock)
 
 		req->length = len;
 		list_del(&req->list);
+		req->zero = (gs_buf_data_avail(&port->port_write_buf) == 0);
 
 		pr_vdebug(PREFIX "%d: tx len=%d, 0x%02x 0x%02x 0x%02x ...\n",
 				port->port_num, len, *((u8 *)req->buf),