driver: prevent reading a free block

when you read the stream device without arming PCAP, you get garbage
which comes from reading uninitialised buffers, to fix that, we need
to wait for the block to contain data

driver/panda_stream.c

@@ -388,7 +388,7 @@ static int wait_for_block(struct stream_open *open)
 {
     struct block *block = &open->blocks[open->read_block_index];
     int rc = wait_event_interruptible_timeout(open->wait_queue,
-        block->state != BLOCK_DMA, HZ);
+        block->state == BLOCK_DATA || block->state == BLOCK_DATA_END, HZ);
     if (rc == 0)
         /* Normal timeout.  Tell caller they can try again. */
         return -EAGAIN;