serial: 8250_dma: terminate correct DMA in tx_dma_flush()

When flushing transmit side DMA, it is the transmit channel that should
be terminated, not the receive channel.

Fixes: 9e512eaa ("serial: 8250: Fix fifo underflow on flush")
Cc: stable <stable@kernel.org>
Reported-by: Wentao Guan <guanwentao@uniontech.com>
Signed-off-by: John Keeping <jkeeping@inmusicbrands.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20250224121831.1429323-1-jkeeping@inmusicbrands.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>