virtio: console: Fix type of 'len' as unsigned int

We declare 'len' as int type but it should be 'unsigned int', as
get_buf() wants it to be.

Signed-off-by: Amit Shah <[e-mail]>
Reported-by: Juan Quintela <[e-mail]>

drivers/char/virtio_console.c

···
 	struct scatterlist sg[1];
 	struct virtio_console_control cpkt;
 	struct virtqueue *vq;
-	int len;
+	unsigned int len;
 
 	if (!use_multiport(port->portdev))
 		return 0;