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.