virtio-serial: Remove redundant check for 0-sized write request

The check for a 0-sized write request to a guest port is not necessary;
the while loop below won't be executed in this case and all will be
fine.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/virtio-serial-bus.c

@@ -91,9 +91,6 @@ static size_t write_to_port(VirtIOSerialPort *port,
     if (!virtio_queue_ready(vq)) {
         return 0;
     }
-    if (!size) {
-        return 0;
-    }
 
     while (offset < size) {
         int i;