board: cavs15: fixed size error that load firmware by script

When load firmware by script, the buffer size and data size are not
same, so specify size when copy data to buffer.

Signed-off-by: Jian Kang <jianx.kang@intel.com>

boards/xtensa/intel_adsp_cavs15/tools/lib/stream_desc.py

@@ -38,7 +38,7 @@ class DmaBuf:
             raise ValueError("Not enough buffer. allocated: %d requested: %d"
                              % (self.alloc_size, size))
         logging.debug("Copying Data to DMA buffer")
-        self.buf[:] = data[:]
+        self.buf[:size] = data[:size]
 
     def free(self):
         if self.mem: