net: wan/fsl_ucc_hdlc: Fix dma_free_coherent() in uhdlc_memclean()

In the Linux kernel, the following vulnerability has been resolved: net: wan/fsl_ucc_hdlc: Fix dma_free_coherent() in uhdlc_memclean() The priv->rx_buffer and priv->tx_buffer are alloc'd together as contiguous buffers in uhdlc_init() but freed as two buffers in uhdlc_memclean(). Change the cleanup to only call dma_free_coherent() once on the whole buffer.